48 CHORDATE ANATOMY 



ventral region where the greater part of the original mass of yolk cells 

 persists. The endoderm surrounds a capacious cavity, the archenteron, 

 whose external opening, the blastopore, is occupied by the yolk plug. The 

 blastopore marks the posterior end of the embryo. The greater part 

 of the original yolk is now in the endoderm. 



The difference between gastrulation in Amphioxus and that in the 

 amphibian is essentially this: in Amphioxus the vegetal hemisphere 

 (prospective endoderm) of the blastula actively moves into the interior 

 of the embryo; in the amphibian the eventual interior position of the 

 endoderm material is due mainly to the enclosing of the yolk-mass by 

 overgrowth (epiboly) carried out by the fold which was initiated by 

 invagination. In Amphioxus the endoderm goes inside; in the amphibian 

 it is put inside by being covered over. Quite clearly the difference is the 

 necessary consequence of the presence of the great mass of inert yolk 

 in the amphibian blastula. 



Fig. 41. — Gastrulation in the pigeon. Section approximately median, showing 

 formation of endoderm by invagination at posterior edge of blastoderm. A, archen- 

 teron; B, blastocoele (cleavage cavity); BP, blastopore; EC, ectoderm; EN, endo- 

 derm; V, vitelline membrane; Y, yolk. Magnified about 100 diameters. (After J. 

 T. Patterson.) 



In Reptiles and Birds. A reptilian or avian embryo whose yolk-mass 

 may be millions of times that of Amphioxus could hardly be expected to 

 carry out a process of gastrulation similar to that of Amphioxus — if, 

 indeed, anything comparable to gastrulation were to be recognized at 

 all. Yet the original single layer of the blastoderm, formed by cleavage 

 (Figs. 36, 37), must somehow give rise to additional layers. The fact 

 is that the blastoderm does at an early period become two-layered. The 

 details of the mode of origin of the second layer differ considerably in 

 various members of the Sauropsida. The significant fact is that the 

 deeper layer (endoderm) results, in part if not entirely, from an inward 

 movement of blastoderm cells at the median region of what proves to 

 be the posterior edge of the blastoderm (Fig. 41). This inward movement 

 may consist in the formation of a small pit, an actual invagination, from 

 whose bottom cells move forward and laterally underneath the original 

 blastodermic layer to become the endoderm. In other cases there is 

 merely an in-turning of the mid-posterior edge of the blastoderm without 

 formation of a complete pocket or invagination. In either case the process 

 is confined to the mid-posterior region of the edge of the blastoderm. 



