Developmental Biology ONLINE!

Cleavage of Microlecithal Eggs

This page shows a series of photographs which follows a microlecithal (oligolecithal) egg from the unfertilized single cell through to the late blastula. Microlecithal eggs have a very small amount of stored yolk, distributed even throughout the cytoplasm (unlike the chicken, which has a large isolated yolk). As a result of this small amount of yolk, these animals show one of two different methods of development. The first is to have a feeding larval stage (as this embryo will have) to meet the nutritional requirements of the embryo, with the second being to have internal development such that the nutritional requirements of the embryo are met by the mother (what we will see later with mammals). These particular slides are whole mounts of starfish embryos. Once again, the use of the "Forward" and "Backward" buttons will allow you to navigate through the microlecithal cleavage. Once you have viewed and assimilated this page (resistance is futile!), move on to microlecithal gastrulation.
To the first slide!

Microlecithal Cleavage - Slides 1 and 2
The unfertilized egg
Later
Later on
The fertilized egg
This picture is of the unfertilized egg. It can be differentiated from the zygote by the presence of a large, conspicuous nucleus (large arrow) with obvious nucleolus (smaller arrow) and by the lack of a fertilization membrane.
This shows the zygote (fertilized cell). It is recognized by the presence of the fertilization membrane (arrows) surrounding it and the peripheral, fluid-filled perivitelline space.
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Microlecithal Cleavage - Slides 3 and 4
The two cell embryo
Later
Later on
The four cell embryo
This is the two cell stage. By this stage, the zygote has completed its first cleavage, which is both equal and holoblastic (i.e. the entire ovum is divided into cells). The division (cleavage) has passed through the animal-vegetal axis, producing two similar blastomeres.
This is the four cell stage. This second cleavage also passes through the animal-vegetal axis, but perpendicular to the first cleavage (described previously). Four equal-sized blastomeres are the end result.
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First slide Top of the page

Microlecithal Cleavage - Slides 5 and 6
The eight cell embryo
Later
Later on
The 32 cell embryo
The eight cell stage. Here, the third cleavage has occurred in the equatorial plane (perpendicular to the first two cleavages and the animal-vegetal axis). Note that the upper four blastomeres (the animal pole) are slightly smaller than the lower four blastomeres (the vegetal pole). NOTE: The sixteen cell stage is not shown - its division is once again vertical, along the animal-vegetal axis.
The thirty-two cell stage is seen here. After the 16 cell stage, the cleavages become more difficult to follow, due to the increasing number of cells and to the division of blastomeres becoming asynchronous. Cleavage continues, forming a mass of cells which organizes itself into the blastula. The lighter area in the centre of the embryo is the beginning of the blastocoel.
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First slide Top of the page

Microlecithal Cleavage - Slides 7 and 8
The early blastula
Later
Later on
The late blastula
This shows the early blastula. With continuing cleavage, the cells in centre begin to lose contact with one another, and a central fluid-filled cavity (the blastocoel) forms. This blastocoel is surrounded by a single layer of cells, forming the hollow sphere know as the blastula.
This shows the late blastula. Like the early blastula, it is characterized by a single layer of cells surrounding the central hollow area - the blastocoel (B). The blastomeres are seen to be smaller and are individually not as obvious. The blastomeres at the vegetal pole (VP) are taller than those at the animal pole (AP), making the vegetal pole appear slightly thicker.
To Gastrulation Backward
First slide Top of the page

To microlecithal gastrulation
To microlecithal gastrulation

This page is part of the Developmental Biology ONLINE! web site.
Special thanks to Hans Christoffersen for the preparation of this page.

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Send comments to:

Dr. Steven Scadding ( scadding@uoguelph.ca ) or Sandra Frombach (sfrombac@uoguelph.ca),

Department of Zoology, University of Guelph, Guelph, Ontario, Canada N1G 2W1.

Last Revision: August 10, 1998