Whole Mount of Chick Blastoderm

AO = area opaca, AP = area pellucida

P = region of thick blastoderm that marks the future posterior region of the embryo; area where gastrulation begins

Arrows = movement of rapidly dividing cells of epiblast layer

Chick Embryo - Primitive Streak Stage

AO = area opaca, AP = area pellucida, P= posterior

The primitive streak is the region where epiblast cells converge (solid arrows) and move through a groove into the blastocoel (dashed arrows). The primitive steak consists of a lightly staining groove bordered on each side by darker staining ridges. Henson's node marks the anterior of the streak and P marks the posterior.

Cross sections through Primitive streak

Key to Upper Cross section: AO = area opaca, . AP = area pellucida, ^^ = remnants of yolk granules, arrows = direction of ingression of embryonic cells

Lower Cross Section shows the three germ layers formed by the process of gastulation.

Terminology

Do you understand the terms used to describe cellular movements in gastrulation in microlecithal, mesolecithal, and megalecithal embryos?

Distinguish between the following terms:

• epiboly

• ingression

• invagination

• involution

Glossary (Use your browser Back button to return to this page)

Visit the University of Wisconsin for an excellent tutorial describing the above and additional terms in Gastrulation: Morphogenetic Movements.