66 STRUCTURE OF THE VERTEBRATES 



bryo, and an outer ring of cells which are beginning to spread 

 around the yolk as extra-embryonic tissue. At the edge of the 

 inner disc a ventral inpocketing of the marginal cells begins, 

 and by continued growth folds under and spreads anteriorly 

 and laterally, paralleling the original blastula cells. The point of 

 inpocketing is homologous with the blastopore. 



Thus the gastrula is composed of two parallel, flat plates of 

 cells, the original plate being the ectoderm, the ventral layer 

 next the yolk being the endoderm. The slight cavity between the 



Blastodisc- 



First 



Second 



Fourth 



Top View of Egg Long. Section 



of Egg 



... .^ /Ectoderm 



Segmentation cavity / 4 rchenteron 

 ^/ Blastopore ,^^^ /.Endoderm 



Yolk 



Cleavage of Blastodisc 



Extra-embryonic 

 membranes Embryo 



/Amniotic 

 fold 



I 



Fig. 24. Early Embryology of the Chick. The egg has a large yolk with a 

 dorsal drop of protoplasm. Cleavage is meroblastic. The longitudinal sec- 

 tions (bottom, left) show gastriilation. The diagrams at the right show the 

 overgrowth of the extra-embryonic membranes, the embryo limited to a 

 small area at the top. 



endoderm and the yolk is the archenteron. It is spread out over 

 the yolk, and only takes shape with the development of the 

 embryo. 



The neural groove appears as a faint line extending anteriorly 

 from the blastopore. The median line of cells sinks while the 

 lateral neural ridges rise toward the dorsal median line, much 

 as in the frog. Synchronously with the above, the notochord 

 makes its appearance as a proliferation of endodermal cells, 

 forming a continuous structure ventral to, and parallel with, the 

 neural groove. 



