1. Gastrulation
Prepared by: W.R. JACINTO

2. Rearrangement of blastomeres to establish multilayered body plan by morphogenetic movements.
Cells forming endodermal and mesodermal organs are brought inside
Cells to form the ectoderm are spread over outside surface
Cells are brought close enough together for inductive interactions

4. Sea urchin Determination of blastomeres
Studies have shown that by the 60-cell stage, most of the embryonic cell fates are specified, but that the cells are not irreversibly committed.

5. Invagination of the vegetal plate to one-half way into the blastocoel

6. Ingression of primary mesenchyme
Ingression from vegetal plate at the late blastula
Cells develop filopodia to test substrates then ingress
Cell disassociate from the monolayer derived from the micromere

7. Invagination Cells move to fill gaps created by ingression
Cells move inward – forms archenteron
Blastophore – opening of the archenteron

8. Archenteron formation - main feature
First formed by invagination and elongation into the blastocoel
Second, presence of secondary mesenchyme at the tip of the archenteron extending filopodia toward the opposite wall of the blastula.
Third, after contact, secondary mesenchyme transform into
Pigment cells, blastocoelar cells, coelomic pouches and circumesophageal musculatiure.

9. Bilaminar wall ruptures to form the oral opening; blastophore becomes the anus.
Skeleton rods arise- leads to formation of pluteus larva.

11. Amphibian

12. Initiated at the future dorsal side, in the region of the gray crescent – marginal region where the animal meets the vegetal
Bottle cells’ pull inside caused invagination
Animal pole cells undergo epiboly
Goosecoid and nogin genes - activated

13. Chordamesoderm – cells passing over dorsal lip
Cells undercut the ventrally – result to formed yolk plug

14. Birds Already in progress at the time of laying
Forms primitive streak – landmark in birds, reptiles and mammal

15. P. streak extends from posterior to anterior
Migrating cells move from dorsal to ventral
Primitive groove, primitive knot or Hensen’s node, primitive ridges
Head process – where notochord is

16. Regression of primitive streak
Migrating cells becomes the (a)pharyngeal endoderm of the foregut (b) head mesenchyme and pre-chordal plate (c) notochord

17. Goosecoid gene in Hensen’s node and Koller’s sickle
As avian gastrulation draws to a close, the ectoderm has surrounded the yolk, the endoderm has replaced the hypoblast, the mesoderm has positioned itself between these regions.

18. Mammals

20. Cell migrating to the node become the notochord
Notochord cells are integrated in the endoderm in mouse