ON THE PROCESS OF GASTRULATION IN CHELONIA. 



253 



Woodcut II. 



knob has by tlie absorption of the anterior part of its floor (indicated 

 by dotted lines) been put in communication with the subgerminal 

 cavity in the yolk. The anterior end of the invagination-cavity is 

 clearly recognisable at the time of the breaking through ; it becomes 

 invisible for a time after that event, but is soon marked out again by 

 the commencing head-fold. Tlie tliick part of the hypoblast (marked 

 with slant lines) is intended to show the extent to which cells from 

 the primitive knob spread themselves. The structures l^ehind the 

 invagination cavity — the yolk-plug, the peristomal mesoblast — have 

 been fully described in ('Ontribs. I. & III. 



When we compare 

 this diagram with tlie 

 well-known one of an 

 amphibian egg (Wood- 

 cut II.) given by Hert- 

 wig, their similarity be- 

 co7nes very striking. 

 .p.Tist-Mesoi.i. The structures dorsal to 

 the line Z— Yin the am- 

 phibian egg can be 

 identified,^ part for part, 

 on the emljryonic shield of the chelonian egg. In homologizing 

 these two eggs a great deal depends upon the view we take as 

 to the nature of the invagination-cavity, the breaking through 

 of the same, and the large subgerminal cavity into which the 

 the invagination cavity opens. I have already made mention of the 

 assumption that the invagination cavity gives rise to the definitive 

 alimentary canal (p. 245). The following considerations will make 

 my views clear and I trust, justify them at the same time : — There 

 is in the chelonian egg a large yolk reservoir A (Woodcut III. 1). 



Y.^lk-Pluff 



