ox THE PROCESS OF GASTliULATION IX CHELOXIA. 267 



VîigiDîition cavity to be equal to tlie width of the shield. In this 

 connection we must renieniher jinother fact which Will Irinos out in 

 another place and which I belie\e myself able to corroborate, that 

 " die gesammte dorsal Lh-darmwand zur Bildung der ClKU'da und des 

 gastralen Mesoderms aufgelrraucht wird" (II Art. p. (il:^).* If the 

 invagination cavity becomes, as Will maintains, really coextensive 

 witii the embryonic shield, it follows from the above-mentioned fact 

 that the lower layer covering the entire ventral surface of the embryo- 

 nic shield is used up for the above-mentioned j)urpose and the gut-hypo- 

 blast (Darm-Entoblast) must come fnjm outside the shield. Ikit this 

 can not be reconciled with the fact whicli I have brought out in the 

 foregoing pa])er and of which there can not be any doubt, that the 

 gut-hypoblast comes from the cells derived from the primitive plate 

 and arises within the embryonic shield. It can not be urged 

 that there are actual differences in this respect between Cistudo 

 which Will has studied and C h el o nia which I have studied, for his 

 Fig. 1) (II Art.) is very much like my Figs. 6a or 7a in the fore- 

 going article, and shows, beyond a shadow of doubt, that in Cistudo as 

 well as Chelonia the gut-hypoblast arises within the embryonic shield. 

 These considerations force me to suspect that possibly there is no 

 great difterence in the actual facts between Cistudo and Chelonia 

 in this matter. 



4. As I have stated in the foregoing article, I was unable to 

 detect in Chelonia any differentiation of the primary and secondary 

 endoderm such as Will and several others describe. 



5. In one place Will does me injustice. On p. 587 (II Art.) 

 he says : '• Die Hedeutung dieses Flächenbildes, von dem ich in 

 Holzschnitt Fig. 7. (my Fig. '2, Contrib. III.) eine einfache Skizze 



* I am iudeliteil to Dr. "W^ill for poiuting ont the inaccuracy of uiy exj^ression on this point 

 undor Heading- 4 of my proliuiinary notice f J Hr(^ Aiiz., VIII. Jahr(j., So. 12J13.). 



