REPRODUCTION 69 



to extend and complete the endodermal sheet produced by the over- 

 growing fold and, at the expense of the blastocoele, to increase the archen- 

 teric space. 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 endo- 

 derm material is effected largely by the overgrowing activity of the 

 prospective ectoderm. 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. 



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, 



Fig. 45. — 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.) 



indeed, anything comparable to gastrulation were to be recognized at all. 

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

 (Figs. 40, 41), 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 vari- 

 ous members of the Sauropsida. In the present connexion the significant 

 fact is that, in all cases so far as known, 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. 45). This inward movement may consist in the forma- 

 tion 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. The endoderm, thus initiated, 

 rapidly spreads over the yolk mass and under the original layer which is 

 now identified as the ectoderm. The growth of the endoderm may be 

 augmented by cells which become detached from the under surface of the 

 outer laver. 



