ON THE PROCESS OF GASTRULATION IN CHELONIA. 251 



8. The future embryo and consequently the defini- 

 tive ulimentary canal is formed entirely within the 

 area covered ventrally by the part derived from the 

 primitive knob. 



Putting the results in another way, they may be summed up as 

 follows : — 



From the epibla.st of the embryonic shield, the 

 EPiBLAST and ITS DERIVATIVES of the future animal is 

 derived. In the region of the primitive plate and its 

 anterior enlargement are produced the invagination- 

 CAVITY (the Archenteron), the yolk-plug, the chorda, 

 the MESOBLAST (botli peristomal nnd gastral), and the 

 DEFINITIVE HYPOBLAST and ITS DERIVATIVES. The primi- 

 tive lower layer forms the wall of the yolk-sac, and 

 contributes to the future animal only in so far as some of 

 its cells are unrecognisably incorporated with the cells 

 of the primitive knob, when the latter spreads itself 

 over the ^^ e n t r a 1 sur fa ce of the e m b r y o n i c shield. 



On the last point, I find myself at variance with Wenckebach 

 who makes the cœnogenetic hypoblast take part in the formation of 

 the anterior ])art of the embryo-body, "namentlich an dem cranialen 

 Wachsthimi von Chorda und gastralem Mesoderm (No. 15, p. 76). 



Theoretical Considerations. 



If we represent the chelonian egg dia grammatically in the light 

 of the facts described in the preceding pages, we shall obtain something 

 like that given in Woodcut I. 



