A

adhesive glands Paired ectodermal thickenings found of the ventral surface of anuran embryos that secrete adhesive mucus. Synonyms: cement glands, mucous glands, oral suckers.

albumin [L. albus - white]. In reference to development, the egg white of birds eggs formed in the oviduct before the addition of the shell. It provides protein and water to the developing embryo and protection from microorganisms.

allantois [Gr. allantos - a sausage]. An extraembryonic membrane formed by an extension of the hindgut into the extraembryonic coelom and consisting of endoderm and splanchnic mesoderm. In birds and reptiles, it fuses with the chorion to form the chorioallantoic membrane which assists in respiration, and in addition it is the site of storage of nitrogenous wastes. In eutherian mammals, it forms the vessels of the umbilical cord and fuses with the chorion to form the embryonic portion of the placenta.

amnion [Gr. amnion - the membrane around the fetus]. An extraembryonic membrane surrounding the embryo and containing the amniotic fluid which protects the embryo from shock and desiccation. It consists of ectoderm and somatic mesoderm.

amniote A species of animal which possesses an amnion. This term is most often used in the plural (amniotes) to refer to reptiles, birds, and mammals collectively.

androgen [Gr. andros - man; Gr. gennan - to produce]. Any male sex steroid associated with the development of male sex organs and secondary male characteristics.

animal hemisphere The half of an oocyte which contains less yolk or the corresponding half of an embryo with the more actively dividing cells.

animal pole The animal hemisphere end of the axis of an oocyte or embryo; the end opposite the vegetal pole.

anterior intestinal portal [L. intestinus - internal; L. porta -a gate]. The opening of the foregut in developing amniote embryos. As development progresses and the gut elongates, the opening moves posteriorly and ultimately becomes part of the yolk stalk.

antrum [Gr. antron - a cave]. The fluid-filled cavity of a maturing ovarian follicle.

apical Referring to the location at the tip, apex or summit.

archenteron [Gr. arche - primitive; Gr. enteron - intestine]. The gut cavity formed by gastrulation in metazoan embryos; formed by infolding of part of the outer surface of the blastula and opening to exterior via blastopore. Synonyms: primitive gut, gastrocoele.

area opaca [L. area - a space; L. opacus - shaded]. The peripheral region of the chick blastodisc surrounding the area pellucida and in direct contact with the yolk. This area develops into two regions.

area opaca vasculosa The inner region of the area opaca where thickenings of splanchnic mesoderm form blood islands.

area opaca vitellina The outer region of the area opaca which initially lacks any blood islands or blood vessels.

area pellucida [L. pellucidus - clear, transparent]. The central region of the blastodisc where the primitive streak forms. It lies directly over the subgerminal cavity and so appears transparent.

ascaroside layer The inner, thin layer of an Ascaris egg shell which holds the first polar body against the underside of the chitinous layer.

atretic follicle [Gr. atretos - not perforated]. The resulting end-product of a degenerated ovarian follicle. The developing follicle may stop maturing at any time during maturation causing atresia, or degeneration of the follicle, its components and the ovum.

atrium [L. atrium - hall] In the embryo, it is the chamber of the heart that receives blood from the sinus venosus and delivers blood to the ventricle.

B

bipinnaria larva In the class Asteroidea, the free-swimming, ciliated stage of larval development.

bivalent A pair of homologous chromosomes closely adjacent to each other formed during the first meiotic division within a primary spermatocyte or oocyte.

blastocoel [Gr. blastos - germ, bud, shoot; koilos - hollow]. The large fluid-filled cavity of the blastula formed by cleavage of the zygote. It may eventually be replaced by the growing archenteron.

blastoderm [Gr. blastos - germ, bud, shoot; Gr. derma - skin]. The divided germinal disc of a megalecithal egg lying on the yolk and composed of three regions: the area pellucida, the area opaca, and the syncytial periblast. Synonym: blastodisc.

blastomere [Gr. blastos - germ, bud, shoot; meros - a part]. Cells produced by cleavage of the zygote.

blastopore [Gr. blastos - germ, bud, shoot; L. porus - a pore or passage]. The opening between the archenteron and outside of the embryo, formed by invagination during gastrulation. In some organisms, it becomes the anus.

blastula (plural: blastulae) [Gr. blastos - germ, bud, shoot]. The multicellular embryo formed as a result of cleavage. In higher animals it contains a fluid-filled cavity called the blastocoele.

blood islands Masses of splanchnic mesoderm that form the first red blood cells and blood vessels. Found in the gut wall of amphibians and the yolk sac of amniotes.

bulbus arteriosus The anterior chamber of the early embryo's heart which functions to connect the ventricle to the ventral aorta. Synonyms: bulbus cordis, conus arteriosus, truncus arteriosus.

C

centrolecithal [Gr. kentron - point, spine, center of a cicle; Gr. lekithos - yolk]. A characteristic of an egg, such as that of an insect, in which the yolk is centrally located.

centromere [Gr. kentron - point, spine, center of a circle; Gr. meros - a part]. The region of a chromosome where two chromatids are joined and where spindle fibers attach during mitosis and meiosis.

chalaza (plural: chalazae) One of a pair of twisted, cords of albumin found at each end of an egg, joining the shell membrane to the yolk and supporting the yolk centrally within the shell.

chiasmata An X-shaped crossing formed by the crossing over of homologous chromosomes during meiosis when genetic material is exchanged.

chitinous layer [Gr. chiton - an outer covering, tunic]. A thick, clear layer of an Ascaris egg shell which is composed of chitin and protein and provides a protective coating located between the fertilization membrane and the ascaroside layer.

chondroblast [Gr. chondrion - cartilage; Gr. blastos - germ, bud, shoot]. A cell which gives rise to cartilage.

chondrocyte [Gr. chondrion - cartilage; Gr. kytos - a hollow vessel]. A mature chondroblast located in a lacunae and surrounded by a matrix. These cells make up the cellular component of cartilage.

chordamesoderm [Gr. chorde - guts, a string; L. chordata - having a notochord or "back-string"] The cellular layer laid down during gastrulation which becomes the prechordal plate, notochord, and dorsal mesoderm of embryo. Functionally, it induces the overlying ectoderm to develop into neural tissue.

chromatophore [Gr. chromatos - the surface of the body, the colour of the skin surface, colour; Gr. phoros - bearing]. A pigment containing cell. These are located in the dermis of the skin as well as in other locations in the body.

cleavage [Anglo-Saxon cleofian - to cut, divide]. The process of cell division in the zygote to form a multicellular embryo. There is usually no growth of cells (blastomeres) at this time so the cells decrease in volume by about 50% with each subsequent division. The embryo is referred to as a blastula at this time.

coelom [Gr. koilomatos - a hollow]. A body cavity lined by mesoderm. It is usually formed through either enterocoelic pouching (in Deuterostomes) or by schizocoely (in Protostomes).

competence [L. competentia - meeting together, agreement, symmetry] The ability or state which renders a cell capable of responding to an inductor or hormone The dorsal ectoderm of amphibian embryos displays competence for neural induction. [P.D. Nieuwkoop: "In my opinion, the central question in embryology is therefore the nature of competence, its formation, climax, and decline." From: R. Gordon, N.K. Bjorklund, P.D. Nieuwkoop. Appendix: Dialogue on embryonic induction and differentiation waves. Int. Review Cytology, 150:373-420].

competent [L. competentia - meeting together, agreement, symmetry]. Displaying competence. (The technical use of the term here follows the normal English sense of being able or fit.) Dorsal neural ectoderm in amphibian embryos consists of competent cells which if induced can form the neural plate.

copulation [L. copulatio - sexual coupling] The sexual act of a male and female, of the same species, coming together for the purpose of delivering and receiving sperm respectively. The term can also refer to sexual behavior that enhances the possibility of fertilization.

corona radiata [L. corona - a crown]. A layer of protective follicle cells derived from the cumulus oophorus surrounding the zona pellucida of an oocyte after ovulation.

corpus albicans (plural: corpora albicantia) [L. corpus - body; L. albicus - white]. The fibrous scar tissue formed as a result of the degradation of the corpus luteum in the ovary.

corpus luteum (plural: corpora lutea) [L. corpus - body; L. lutum - as a colour, clay-yellow]. A temporary gland formed from a ruptured Graafian follicle and the surrounding thecal cells after ovulation. If fertilization occurs, the corpus luteum persists and continues to secrete progesterone. If fertilization does not occur, the corpus luteum degenerates.

cortex [L. cortex - outer shell, bark]. The outermost layer of an organ or the outer, less fluid cytoplasm of an egg.

cumulus oophorus [L. cumulus - a little mound; Gr. oophoros - bearing eggs]. A thickening of follicular cells that encloses the ovum and projects from the stratum granulosum into the antrum.

cytokinesis [Gr. kytos - a hollow vessel, now often taken to mean a cell; Gr. kinesis - movement] The division of the cytoplasm of a cell after the division of the nucleus.

cytoplasm [Gr. kytos - a hollow vessel, now often taken to mean a cell; Gr. plasma - anything formed or moulded]. A jelly-like material bounded by the plasma membrane of the cell, containing organelles (excluding the nucleus) and providing a medium for metabolic activities.

D

decidua [L. deciduus - deciduous, falling off]. The material of the endometrium of the mammalian uterus during pregnancy.

decidua basalis [NL. basilaris - pertaining to or situated at the base] Maternal portion of the decidua that comes in contact with the embryonic chorionic villi and underlies the conceptus.

decidua capsularis [Gr. capsula - a box, case] Portion of the decidua surrounding the embryo conceptus where it projects into the uterine lumen. It will eventually fuse with the decidua parietalis.

decidua parietalis [L. parietalis - pertaining to walls] Portion of the decidua lining the rest of the uterus.

deciduate placenta [L. placenta= Gr. plakous - a small, round, flat cake]. Type of placenta where the lining of the uterus is shed along with the placenta, after birth has taken place. This is due to the extensive penetration of the chorionic villi into the endometrium. There are two types of deciduate placenta which reflect the arrangement of the chorionic villi: zonary and discoidal.

delamination [L. de - prefix meaning down, away, off; L. lamina - plate, leaf, layer]. The process by which a cell layer splits into two separate layers.

dermis [Gr. derma - skin, leather]. The lower layer of the skin beneath the epidermis consisting of connective tissue. It may contain hair follicles and sweat and mucous glands.

deuterostomes A group of Metazoans that exhibit indeterminate, radial cleavage and that develop a mouth independent of, and at some distance from, the blastopore. (Echinoderms and Chordates and related minor phyla).

diakinesis [Gr. dia - apart; Gr. kinesis - movement]. The last stage of prophase 1 of meiosis, leading to metaphase of the first meiotic division. The chromosomes are at maximum condensation and the nuclear membrane breaks down.

diplotene [Gr. diplo - double; Gr. tainia - ribbon]. The fourth stage of prophase 1 of meiosis, following the pachytene stage, in which there is continued shortening and thickening of the chromosomes. Final gene transcription is completed, after which diakinesis occurs.

E

ectoderm [Gr. ektos - outside, without; Gr. derma - skin, leather]. The outermost germ layer of the embryo that develops during gastrulation. It will form the nervous system, epidermis, and their accessory structures.

embryo [Gr. embryon]. The stage in a developing organism after cleavage has occurred and before hatching or birth.

endocardium [Gr. endon - within; Gr. kardia - The inner layer of the heart rudiment that arises from splanchnic mesoderm and fuses with the epimyocardium to form the heart wall.

endochondral ossification [Gr. endon - within; Gr. chondrion - cartilage; L. ossiculum - bone]. The process of bone formation from cartilage.

endoderm [ Gr. endon - within; Gr. derma - skin, leather] The innermost germ layer of the embryo that develops during gastrulation. It will form the lining of the respiratory and digestive organs and their derivatives.

endometrium [Gr. endon - within; Gr. metra - the uterus]. The lining of the uterus, the structure of which reflects the cyclical stages of the menstrual (primates) or estrous (other mammals) cycle.

endothelium [Gr. endon - within; Gr. thele - nipple]. The layer of cells lining the lumen of blood vessels and heart. It is derived from mesoderm.

enterocoelic pouches [ Gr. enteron - intestine; Gr. koilos - hollow]. Outpocketings of the archenteron (primitive gut) of the gastrula which form the mesodermal somites later in development. The cavities within the pouches will form the body cavity. Found in animals such as Amphioxus and many echinoderms. Synonym: coelomic sacs.

epiblast [Gr. epi - on, upon; Gr. blastema - an outgrowth]. The outer layer of the amniote blastoderm. It gives rise to cells of all three germ layers.

epiboly [Gr. epi - on, upon; Gr. bole - a throw]. The movement of the animal pole blastomeres over and encompassing the vegetal hemisphere blastomeres that occurs in the embryo at the beginning of gastrulation in some amphibians and teleosts.

epidermis [Gr. epi - on, upon; Gr. dermis - skin, leather]. The outermost skin layer, derived from ectoderm.

epididymis [Gr. epi - on, upon; Gr. didymos - testis]. A coiled structure found along the posterior border of the testis where mature spermatazoa are received, stored and eventually dispensed into the vas deferens for transport out of the body.

epimyocardium [Gr. epi - on, upon; Gr. myo - muscle; Gr. kardia - heart] The outer, thicker layer of the heart rudiment which arises from splanchnic mesoderm and fuses with the endocardium to form the heart wall.

epiphysis [Gr. epi - on, upon; Gr. physis - growth]. An evagination of the prosencephalon (forebrain) which later forms the pineal gland.

epithelium [Gr. epi - on, upon; Gr. thele - nipple]. A tissue consisting of one or more layers of tightly associated cells forming a covering or lining; may be derived from endoderm, mesoderm, or ectoderm.

estradiol A female sex hormone secreted by the ovary which, in conjunction with progesterone, functions to maintian the uterine mucosa in case fertilization occurs.

estrous cycle [Gr. oistros - a strong desire]. The female reproductive cycle of most mammals (not primates). It consists of four stages: proestrus, where the uterine lining begins to thicken; estrus or the "heat" phase of sexual receptivity when ovulation occurs; metestrus, where the corpus luteum forms and secretes hormones to maintain the uterine lining and prevent further ovulation; and diestrus, where if fertilization does not occur, the uterine lining is reabsorbed.

extracellular matrix [L. extra - beyond, on the outside; L. cellula - storeroom, chamber; L. matricis - place where anything is generated]. The component of connective tissue surrounding cells and made up of the amorphous ground substance and fibrous proteins (mainly collagen fibres). The amorphous ground substance consists primarily of glycoproteins and proteoglycans.

F

fascicle [L. fasciola - a bundle, band]. A bundle of nerve or muscle cells.

fertilization [L. fero - to bear]. The formation of a single-celled zygote through the union of male and female gametes that occurs in sexual reproduction.

fertilization membrane A membrane formed in some aquatic species from the vitelline membrane and cortical granules immediately after sperm penetration into the ovum. It is thought to prevent polyspermy by making the egg impenetrable to other sperm.

fibroblast [L. fibrilla - a fibre, filament]. A cell found in ordinary connective tissue that produces fibres.

G

gametogenesis [Gr. gamos - a marriage; Gr. genesis - origin, descent]. The formation of male and female sex cells or gametes (spermatozoa and ova, respectively) from germ cells.

gastrula (plural: gastrulae) [L. gastrula - the belly, stomach] The early embryonic stage following the blastula stage during which cell migrations form distinct germ layers. The primitive gut (archenteron) is formed and opens to the exterior through the blastopore.

gastrulation [L. gastrula - the belly, stomach] The process in embryonic development whereby the three primary germ layers are laid down and the gut formed through complex cell migrations.

germ cell [L. germinis - bud, offshoot] A cell found within the gonads that gives rise to a gamete.

germinal disc [L. germinis - bud, offshoot]. In megalecithal eggs, the small area on the fertilized egg to which cleavage is restricted. The blastodisc results from meroblastic cleavage and develops into the embryo.

germinal epithelium [L. germinis - bud, offshoot] The epithelium that covers the surface of a mammalian ovary.

germinal vesicle [L. germinis - bud, offshoot; L. vesicula- bladder]. The greatly enlarged nucleus of an oocyte.

germ layers [L. germinis - a bud, offshoot]. The three major cellular layers (ectoderm, mesoderm, endoderm) which are laid down in the embryo during gastrulation. Each layer is the precursor of different tissues and organs.

germ ring The advancing ring of animal pole micromeres migrating over the vegetal hemisphere in amphibian embryos.

Graafian follicle [Regnier de Graaf, Dutch physician and anatomist, 1641-1673; L. folliculus - a small bag] A follicle found in mammalian ovaries that is mature and ready to release an ovum. After ovulation, it becomes a corpus luteum.

H

Haversian system [Clopton Havers, British physician and anatomist, 1650-1702] The basic structural arrangement of compact bone consisting of: a Haversian canal which contains nerves and blood supply, lamellae which are concentric rings of matrix (produced by osteocytes within lacunae), and canaliculi which provide a link between osteocytes.

Hensen's node [Viktor Hensen, German anatomist and physiologist, 1835-1924] The thickened area at the anterior end of the primitive streak through which cells invaginate to form the notochord. It is homologous to the dorsal lip of the blastopore in amphibian embryos. Synonym: primitive knot.

histology [Gr. histos - a web; Gr. logy - to give an account]. The microscopic study of cells and tissues and their organization into organs.

holoblastic cleavage [Gr. holos - whole, entire]. The pattern of cleavage in which the entire ovum is divided into cells. It is also called complete cleavage and is usually found in embryos with little to moderate amounts of yolk.

hypoblast [Gr. hypo - under, beneath; Gr. blastos - germ, bud shoot]. The inner layer of amniote blastoderm. Its cells usually become the endoderm of the extraembryonic membranes.

hypophysis [Gr. hypo - under, beneath; Gr. physis - growth] Note that this term may be used in two different manners. It may be used to describe an embryonic portion of a developing gland, as in, 1)  The invagination of the stomodeum which fuses with the infundibulum to form the pituitary gland. Synonym: Rathke's pocket. It may also describe a complete adult gland, as in, 2)  An endocrine gland formed from an ingrowth from the stomodeum (Rathke's pocket) and the infundibulum. Synonym: pituitary gland.

I

indeciduate placenta [L. in - prefix meaning not, without; L. deciduus - deciduous, falling off; L. placenta - a small round flat cake] Type of placenta where the uterine lining is not shed with the placenta due to the "looser" connection of the chorionic villi to the endometrium. There are three types of indeciduate placenta which reflect the arrangement of the chorionic villi: diffuse, intermediate and cotyledonary.

induction [L. in - prefix meaning in, into, within, upon, towards; L. duco - to lead]. The developmental process in which the fate of a group of cells is determined by interaction with another group of cells. One group (the inducing cells) controls the differentiation of the other group (the induced cells).

infundibulum [L. infundibulum - a funnel]. 1) Anatomically, any funnel-shaped passage. 2) Embryologically, a ventral evagination of the floor of the diencephalon that forms the posterior lobe of the pituitary gland.

ingression [L. in - prefix meaning in, into, within, upon, towards; L. gressus - a step, course]. Process by which cells migrate from the surface layer of the embryo into the interior as individual cells.

interstitial cell [L. inter - between, among; L. sistere - to set]. A cell that is located in the spaces of a tissue, in particular, between follicles of the ovary and between seminiferous tubules of the testis. In the testes these cells produce testosterone: the male sex steroid.

intramembranous ossification [L. intra - inside; L. membrana - the skin of the body]. The process of bone formation from mesenchyme. This process typically occurs in the flat bones of mammalian crania.

invagination An inward folding of a layer of cells forming an interior pocket.

involution [L. in - prefix meaning in, into, within, upon, towards; L. volvo - to roll]. A process of cellular migration involving movement of a sheet of cells over an edge and then spreading inwards in the opposite direction. This occurs, for example, during amphibian gastrulation.

isolecithal [Gr. isos - equal, similar; Gr. lekithos - yolk]. The equal distribution of yolk throughout an egg.

J K

karyokinesis [Gr. karyon - nucleus; Gr. kinesis - movement]. The division of a cell nucleus into two daughter nuclei each having an identical complement of chromosomes. Synonym: mitosis.

L

lacuna (plural: lacunae) [L. lacuna - ditch, pit]. A small space. More specifically: 1) The cavity enveloping and occupied by chondrocytes and osteocytes; 2) The spaces which develop in the placenta and which will later fuse to form the intervillous lake.

leptotene [Gr. leptos - slender; Gr. tainia - ribbon]. The earliest stage in prophase 1 of of meiosis, when the thread-like chromosomes begin to condense and become visible.

liver diverticulum Endodermal rudiment of the liver and associated glands and ducts.

lumen A term used when referring to the cavity or passageway within a tubular structure (ex. the interior space of a seminiferous tubule).

M

macromeres [Gr. makros - large; Gr. meros - part] Large blastomeres found in the vegetal hemisphere of eggs formed by unequal cleavage (ex. the large blastomeres of the vegetal pole of the amphibian embryo).

matrix (plural: matrices). The extracellular substance in which cells are embedded.

medulla [L. medulla - middle, innermost part]. A term used to describe the innermost part of an organ (ie. the brain, ovary or kidney). For example, in the ovary the middle portion of connective tissue that does not contain follicles.

megalecithal [Gr. megas - great; Gr. lekithos - yolk]. Containing a large quantity of yolk.

meiosis [Gr. meiosis - reduction]. Two consecutive nuclear divisions whereby the resultant daughter nuclei contain half the chromosome complement of other somatic cells; occurs at time of gametogenesis. Synonym: reduction division.

meninges [L. meninx - membrane]. Membranes surrounding brain and spinal cord that have a protective function.

menstrual cycle [L. menstrualis - monthly]. The reproductive cycle of female primates. The cycle consists of four stages: menstrual, where the uterine lining is shed; proliferative, where the endometrium begins to thicken; secretory, where the endometrium thickness is maintained and so becomes suitable for implantation; and ischemic, where if fertilization does not occur, the endometrium begins to break down.

meridional [L. from medius - middle]. In reference to cleavage, the splitting of the fertilized egg cell along a vertical plane at right angles to the equator.

meroblastic cleavage [Gr. meros - part; Gr. blastos - germ, bud, shoot]. The pattern of cleavage in which only part of the ovum (the animal pole) is divided into cells. It is also called incomplete cleavage and is usually observed in embryos with large amounts of yolk.

mesencephalon [Gr. mesos - middle; Gr. kephale - a head]. The mid region of the developing embryonic brain. Synonym: midbrain.

mesenchyme [Gr. mesos - middle; Gr. enchyma - an infusion, something poured in]. Embryonic connective tissue derived from mesoderm and neural crest cells. It later forms predominantly connective tissues, but may also differentiate into epithelial, muscular and nervous tissues.

mesendoderm A tissue layer that gives rise to both mesoderm and endoderm.

mesoderm [Gr. mesos - middle; Gr. derma - skin] The middle germ layer, formed in the embryo during gastrulation. In early organogenesis, three regions of mesoderm are evident: the epimere (= dorsal mesoderm) will form somites; the mesomere (= intermediate mesoderm, = nephrotome) will form the excretory system; and the hypomere (= lateral plate mesoderm) delaminates into the inner splanchnic and the outer somatic mesoderm.

mesolecithal [Gr. mesos - middle; Gr. lekithos - yolk] Containing a moderate amount of yolk.

mesonephros [Gr. mesos - middle; Gr. nephros - the kidneys]. The second excretory organ formed in the embryo, caudal to the pronephros. The pronephric ducts become mesonephric or Wolffian ducts. This kidney is fully functional in adult amphibians and fish. The duct portion becomes the epididymis of the testes in amniotes.

mesovarium [Gr. meso - middle; L. ovarium - ovary]. A supporting mesentary (membrane) from the uterus that holds the mammalian ovaries in place.

metanephros [Gr. meta - after, behind, later in time; Gr. nephros - the kidneys]. The third and permanent excretory organ formed in embryonic reptiles, birds, and mammals, caudal to the mesonephros. The ureter develops from the mesonephric duct.

Metazoa [Gr. meta - after, beyond; Gr. zoon - animal]. The Subkingdom of the Kingdom Animalia that includes all mulitcellular animals developing from embryos and possessing differentiated cells which form into tissues.

microlecithal [Gr. mikros - small; Gr. lekithos - yolk]. Containing a small amount of yolk. Synonym: oligolecithal.

micromeres [Gr. micro - small; Gr. meros - part]. Small blastomeres found in the animal hemisphere during unequal cleavage of a zygote. They are the most actively dividing cells.

mitosis [Gr. mitos - thread]. The process of nuclear division forming two daughter nuclei each having the identical complement of chromosomes as the parent cell.

morphogenesis [Gr. morphe - shape, form; genesis - origin, descent]. The development of form and structure in a cell or aggregation of cells.

morula [L. morus - mulberry]. The early stage of a blastula in which the solid mass of blastomeres resembles a mulberry in appearance. This occurs immediately before the blastocoel appears, forming the blastula.

myoblast [Gr. mys - muscle]. An embryonic muscle fibre forming cell.

N

nephrostome [Gr. nephros - the kidneys; Gr. stomatos - mouth]. The opening of a pronephric tubule to the coelom.

nephrotome [Gr. nephros - the kidneys; Gr. tome - a section]. A stalk-like connection between somites and lateral plate mesoderm which anteriorly forms the pronephos and posteriorly forms mesenchyme which develops into the mesonephros and metanephros. Synonyms: intermediate mesoderm, mesomere, nephromere.

neural crest [L. neuralis - nerve]. A group of cells found at the edges of the neural plate and lying above the neural tube after its formation. They migrate throughout the embryo to form ganglia, pigment cells, parts of gill arches, etc.

neural groove [L. neuralis - nerve]. A median longitudianl groove which is formed by the rolling up of the neural plate.

neural plate [L. neuralis - nerve]. A median, dorsal thickening of ectoderm from which the neural tube and neural crest will develop. Synonym: medullary plate.

neural tube [L. neuralis - nerve]. A tube of tissue formed by a thickening and rolling up of the neural plate during embryonic neurulation. It will later form the brain and spinal cord of the animal.

neurocoele [Gr. neuron - nerve; Gr. koilomatos - a hollow]. The cavity or lumen of the neural tube.

neuroglia [Gr. neuron - nerve, Gr. glia - glue]. Cells of the central nervous system which provide support, protection and insulation for neurons.

neuromeres [Gr. neuron - nerve, Gr. meros - a part]. Segments of the brain which likely reflect the segmental origins of the vertebrates.

neuropore [Gr. neuron - nerve; Gr. poros - pore]. The anterior or posterior openings of the neural tube that will close at a later stage of development.

neurulation The process of the formation of the neural plate and subsequently, the neural tube.

notochord [Gr. noton - back; Gr. chorde - guts, string]. Rod-like structure originating from mesoderm and lying dorsal to the gut, defining the anterior/posterior axis in the developing embryo. It provides skeletal support during early development.

O

oocyte [Gr. oon - an egg; Gr. kytos - a hollow vessel, modern usage: cyt- denotes a cell]. A female germ cell that must undergo two meiotic divisions to become a mature ovum.

oogenesis [Gr. oon - an egg; Gr. genesis - origin, descent] The process in females whereby a mature ovum is formed from an oocyte.

oogonium [Gr. oon - an egg; Gr. gone - seed] Germ cell in the ovary that gives rise to oocytes through repeated mitotic divisions.

organogenesis [Gr. organon - organ; Gr. genesis - origin, descent]. Formation of organs in the developing embryo.

osteoblast [Gr. osteon - bone; Gr. blastos - germ, bud, shoot]. A cell which produces bone matrix. An osteoblast will become an osteocyte once it becomes embedded within the matrix of the bone.

osteoclast [Gr. osteon - bone; Gr. klastos - broken in pieces]. A multinucleated cell that breaks down bone through reabsorption.

osteocyte [Gr. osteon - bone; Gr. kytos - a hollow vessel, modern usage: cyt- denotes a cell] A mature bone cell derived from an osteoblast and located within a lacuna.

ovarian follicle [L. ovarium - ovary; L. folliculus - a small bag] The layers of epithelial cells that surround a developing mammalian oocyte. The process begins with the primary follicle and ends with the Graafian follicle, which contains the mature ovum ready for release during ovulation.

ovarian stroma [L. ovarium - ovary; Gr. stroma - something laid out to build or lie upon]. The structural framework of the ovary that is formed by connective tissue and smooth muscle.

ovary [L. ovarium - ovary] The female gonad: where primary oocytes develop into mature eggs through oogenesis. It is also responsible for the secretion of several female sex hormones (ie. estradiol and progesterone).

ovulation The process whereby a Graafian follicle ruptures and releases a mature oocyte from the ovary.

ovum [L. ovulum - egg]. Mature female gamete or egg cell.

P

pachytene [Gr. pachytes - thickness; Gr. tainia - ribbon]. The third stage in prophase 1 of meiosis in which there is further shortening and thickening of the bivalents. The chromosomes split into chromatids, allowing crossing over to occur and hence the formation of chiasmata. This continues in the following stage of diplotene.

pericardial coelom [Gr. peri - around; Gr. kardia - heart; Gr. koilomatos - a hollow]. The cavity that surrounds the heart and is bounded by the pericardium.

perivitelline space [Gr. peri - around; L. vitellus - yolk of an egg]. The fluid-filled area formed between an external membrane of the egg (e.g. fertilization membrane, ascaroside layer, zona pellucida, etc.) and the plasma membrane.

placenta [L. placenta - a small round flat cake] The organ found in the uterus of pregnant mammals (and other viviparous species) composed of maternal and fetal tissues in close contact which provides nourishment and gaseous exchange for the fetus.

placentome The structure involved in connecting the fetal and maternal tissues consisting of a cotyledon and a caruncle in the cotyledonary placenta. The cotyledons or chorionic villi are of fetal origin and "plug into" the caruncles or receptacles in the maternal uterine wall.

polar body [Gr. polos - a pole; L. corpus - body]. A minute cell formed during the first and/or second meiotic divisions of oogenesis that contain a nucleus but very little cytoplasm and which eventually degenerate.

polyspermy [Gr. poly - many]. The event in which two or more spermatazoa penetrate and fertilize an ovum.

prechordal plate A mass of chordamesodermal cells located anterior to the notochord which will form head mesenchyme.

primary oocyte During oogenesis, the immature egg cell in which the first meiotic division begins, leading to the production of the first polar body and the secondary oocyte.

primitive streak [L. primitus - at first]. Longitudinal cleft formed on surface of amniote blastodisc by a convergence of cells. Gastrulation begins as epiblast cells migrate towards and into the streak.

primordial follicle [L. primordialis - original]. An immature ovarian follicle that will develop into successively more mature follicles as the oocyte matures.

proamnion [Gr. pro- - a prefix meaning before, in front of; Gr. amnion - the membrane around the fetus]. Region anterior to developing avian embryo, surrounding the head; contains only endoderm and ectoderm. It will later become invaded with mesoderm and become incorporated into the amnion.

progesterone A female sex hormone secreted by the corpus luteum that, in conjunction with estradiol, functions to prepare and maintain the uterine mucosa for the reception, implantation, and development of a fetus if fertilization occurs.

pronephros [Gr. pro- - a prefix meaning before, in front of; Gr. nephros - the kidneys]. The initial excretory organ found in developing vertebrate embryos. It is functional in larval amphibians and fish. It is later replaced by the mesonephric kidney.

pronucleus [Gr. pro- - a prefix meaning before, in front of; L. nux - nut]. The haploid, or reduced, nucleus of a gamete (ie. sperm and egg). The reduced nuclei of the sperm and egg join together in the fertilized ovum to form the zygote.

prosencephalon [Gr. pros - forward, in advance of; Gr. kephale -a head]. The most anterior brain region of developing embryos which later divides into the telencephalon (most anterior brain region) and diencephalon. These regions later differentiate into the cerebral hemispheres, thalamus and hypothalamus and optic vesicles. Synonym: forebrain.

protostomes [Gr. protos - first, primary; Gr. stomatos - mouth]. A group of Metazoans that exhibit determinate, spiral cleavage and that develop a mouth from the blastopore. (ex. Nematodes, Flatworms, Molluscs).

Protozoa [Gr. protos - first, primary; Gr. zoon - animal]. The Subkingdom of the Kingdom Animalia, in older classification systems, that includes all unicellular organisms that lack differentiated tissues and that primarily reproduce through asexual means, although conjugation does occur.

Q R

Rathke's pocket or pouch [Martin Rathke, German anatomist 1793-1860] An evagination of the stomodeum that forms part of the pituitary gland (the adenohypophysis or anterior lobe of the pituitary gland). Synonym: hypophysis (Definition 1).

renal corpuscle [L. renes - the kidneys; L. corpusculum - a body]. The filtration unit found in the cortex of the kidneys and made up of Bowman's capsule and a glomerulus.

rhombencephalon [Gr. rhombos - whirling, turning; Gr. kephale -a head]. The most posterior region of the brain of the developing embryo which is divided into the more anterior metencephalon and the more posterior myelencephalon. It later differentiates into the cerebellum, medulla oblongata, and pons (in mammals). Synonym: hindbrain.

S

Seessel's pocket [Albert Seessel, American embryologist and neurologist, 1850-1910]. The anterior projecting portion of the foregut anterior to the stomodeum which will later form part of the mouth. Synonym: preoral gut.

seminiferous tubules [L. semen - seed; L. ferre - to bear]. The tightly coiled tubules found within the testes of vertebrates containing germ cells that are differentiated into spermatozoa.

serosa [L. serum - whey]. 1) In histology, the term is used to mean a serous membrane such as the peritoneum, pleura, and pericardium. A serous membrane is composed of a layer of epithelium with a thin layer of connective tissue. Synonym: tunica serosa. 2) In embryology, the term is used to mean the most exterior extraembryonic membrane. It consists of ectoderm and somatic mesoderm (somatopleure) and encloses all other extraembryonic membranes including the amnion, allantois, and yolk sac. Synonym: chorion.

Sertoli cell [Enrico Sertoli, Italian histologist 1842 - 1910]. A type of cell found within seminiferous tubules of the testes, a nutritive cell to which spermatids attach for the duration of spermatogenesis until the mature spermatozoa is formed. Sertoli cells form the blood-testis barrier. Synonym: nurse cell.

sinus terminalis [L. sinus - a bay of the sea, a curve; L. terminalis - belonging to a boundary]. The vein encircling the area vasculosa.

somatic mesoderm [Gr. somatikos - of or for the body]. The cellular layer on the external side of the coelom; formed from the delamination of lateral plate mesoderm. In conjunction with somites, it will later form body wall and limbs.

somatopleure [Gr. somatikos - of or for the body; Gr. pleura - the side]. The membrane composed of ectoderm and somatic mesoderm. It forms part of body wall as well as the chorion and amnion.

somites [Gr. somatikos - of or for the body]. Segmented mesodermal blocks located on either side of the developing spinal cord which arise from the dorsal mesoderm. Somites will differentiate into: the sclerotome which forms part of the axial skeleton, the myotome which forms muscle and the dermatome which forms the dermal layer of the skin.

spermatid [Gr. spermatos - seed, semen]. Haploid cell in male testes, formed from spermatocyte by meiosis and will differentiate into a spermatozoon.

spermatocyte [Gr. spermatos - seed, semen; Gr. kytos - a hollow vessel; modern usage cyt- denotes a cell]. An immature diploid germ cell found in the testis that arose from a spermatogonium and that will undergo two meiotic divisions to become four spermatids.

spermatogenesis [Gr. spermatos - seed, semen; Gr. genesis - origin, descent].The process in males whereby mature spermatozoa are formed from spermatocytes.

spermatogonia [Gr. spermatos - seed, semen; Gr. gone - seed]. Diploid germ cells in the testis of males that gives rise to spermatocytes through repeated mitotic divisions.

spermatozoon (plural: spermatozoa) [Gr. spermatos - seed, semen; Gr. zoon - animal]. A mature haploid male gamete. Synonym: sperm cell.

spermiogenesis The process of differentiation by which haploid spermatids are transformed into mature spermatozoa.

spindle fibers A collection of microtubules attached to a centromere during mitosis and meiosis that are responsible for the movement of the chromosomes to opposite poles.

splanchnic mesoderm [Gr. splanchnon - entrails, viscera]. The cell layer on the internal side of the coelom; formed by the delamination of lateral plate mesoderm. Later forms the heart, part of the circulatory system and many other structures.

splanchnopleure [Gr. splanchnon - entrails, viscera; Gr. pleura - the side]. The cellular layer consisting of splanchnic mesoderm and endoderm. It will later form part of the body wall in the embryo as well as the yolk sac and allantois.

stem cell [L. cella - compartment]. An undifferentiated cell that is mitotically active in either replacing dying body cells (somatic stem cell) or producing gametes (germinal stem cell).

stratum corneum [L. strata - a layer; L. corneus - horny]. The outer layer of the epidermis of vertebrates. The layer is keratinized for protection from desiccation and foreign agents.

stratum granulosum [L. strata - a layer; L. granulum - a small particle or grain]. A layer of follicle cells that line the theca of a maturing ovarian follicle.

subcephalic pocket The cavity formed beneath the embryonic head as the head fold develops.

T

telolecithal [Gr. telos - end; Gr. lekithos - yolk]. Exhibiting a pattern of yolk distribution where the yolk is confined to one end of the egg, usually the vegetal pole.

testicular cyst [Gr. cysto - a sac or bladder]. Found in the testicular lobes of grasshoppers (Insecta, Orthoptera), a membrane bound structure containing a group of germ cells that are all at the same stage of spermatogenesis.

testis (plural: testes) [ L. testis - a witness, testicle]. The male gonad; where mature spermatozoa develop within the seminiferous tubules (mammals) or sperm lobes (insects) . Synonym: testicle.

theca externa [L. theca - a case for something; L. externus - on the outside]. A layer of fibrous connective tissue situated on the outer surface of a maturing ovarian follicle, covering the theca interna.

theca interna [L. theca - a case for something; L internus - on the inside]. A layer of vascularized connective tissue situated between the stratum granulosum and the theca externa in maturing ovarian follicles.

thyroid gland [Gr. thyreoeides - sheild form]. An endocrine gland in the throat region which is responsible for controlling metabolism and growth. It is formed from a ventral diverticulum of the endoderm of the pharynx at the level of the second pharyngeal arch.

trophoblast [Gr. trophos - one who feeds; Gr. blastos - germ, bud, shoot]. The outer layer of the conceptus which makes direct contact with endometrium. It develops into the fetal portion of placenta and is made up of two layers: the cytotrophoblast - the inner layer, and the syncytiotrophoblast - the outer layer.

U

uterus [ L. uterus - womb]. The muscular female organ that houses and nourishes the developing embryo or fetus. For some vertebrates it is also the place where the egg shell is formed.

V

vacuole [L. vacuus - empty; L. -ole - diminutive ending]. In a cell, a membrane bound compartment separating material from the cytoplasm.

vas deferens [L. vas - a canal carrying fluid; L. deferens - carrying away] A connecting duct in males between the epididymis of the testis and the urethra or cloaca; formed from the mesonephric duct. Synonym: ductus deferens.

vegetal hemisphere The half of an oocyte with more yolk or the corresponding half of an embryo with the slower dividing yolky cells.

vegetal pole The vegetal hemisphere end of the axis of an oocyte or embryo; the end opposite the animal pole.

vestigial [L. vestigium - a footprint, a track, a trace]. Any structure or organ that is reduced in size or function when compared to a previous embryonic stage or similar structure in related organisms or in the fossil record.

vitelline [L. vitellus - yolk of an egg]. Referring to the yolk of an egg or the ovum.

vitelline circulation [L. vitellus - yolk of an egg]. The embryonic circulatory system supplying blood to the yolk sac via vitelline arteries and its return to embryo through the vitelline veins.

vitelline membrane [L. vitellus - yolk of an egg]. A thin non-cellular membrane secreted by the oocyte and follicular cells that surround an oocyte and that will form the fertilization membrane if sperm penetration occurs.

W X Y Z

yolk [ME yolke - the yolk, the yellow part]. Nutritive stores found within the ovum cytoplasm consisting of proteins and fats.

yolk plug A mass of large, yolky endoderm cells that fill the blastopore of an amphibian gastrula during its late stages.

yolk sac An extraembryonic membrane enclosing and absorbing the yolk of amniote embryos. It is formed from a layer of endoderm and a layer of splanchnic mesoderm (splanchnopleure).

zona pellucida [L. zona - a girdle; L. pellucidus - to shine through]. A translucent non-cellular membrane surrounding a mammalian egg through which sperm must penetrate in order for fertilization to occur.

zooplankton [Gr. zoon - animal; planktos - wandering]. Minute aquatic animals that drift freely in the plankton, feeding mainly on phytoplankton (plant-like organisms) and having no locomotory structures.

zygote [Gr. zygotos - yoked together]. The fertilized ovum which results from the fusion of the male and female pronuclei (before the first cleavage division).

zygotene [Gr. zygotos - yoked together; Gr. tainia - ribbon]. The second stage in prophase 1 of meiosis, characterized by the synapsis, or pairing, of homologous chromosomes to form a bivalent.