ON THE PROCESS OF GASTRULATIOX IN CHELONIA. ^45 



presents now n smootli compact appearance. Tlie head-fold and the 

 chorda -groove have ah'eady Ijecome conspicuous. 



As the head-fold, formed well within the edg^ of the emhryonic 

 shield, marks the anterior end of tlie emhryo, and therefore of the 

 archenteron or the adidt alimentary canal exclusive of the stomod^eum ; 

 as the primitive kn<:>h marks the posterior end of the emhryo ; and 

 as the lateral Iwdy-wall is formed from the lateral folds, also arisen 

 within tlie embryonic shield, we are justified in coming to the very 

 important conclusion that the body of the future embryo and 

 consequently the definitive alimentary canal is formed 

 entirely within the area covered ventrally by cells derived 

 from the primitive knob. This speaks in fli vor of the assump- 

 tion that the invagination cavity is the archenteron and gives rise 

 to the fiitare alimentary canal. I sliall discuss farther on how we 

 ought to regard the breaking through of the invagination-cavity and 

 the gradual sj)reading of the cells of the primitive knob over the 

 ventral face of the embryonic shield. 



The reason wliy the advancing edge of the primitive knob is 

 marked by a zone of network is probably, T tliiid^, that sucli a 

 structure allows free and easy access of the nutritive liquid of the 

 yolk to the deeper parts of the tissue. 



The network sucli as is here described, has been seen many 

 times before. For instance, Ishikawa and 1 noticed it in a 

 Tri onyx blastoderm (Fig. l/> of Contrilj. I) without knowing its 

 significance. Again, Fig. 10. C^ontrib. Ill represents the same 

 thing in cross-section in an embryo of Clem my s . Mehnert (No. 8) 

 gives beautiful ilhistrations of stages showing the network, in his 

 Figs. 4-13. He, however, gives an explanation of it which is at 

 utter variance with the one given above, for according- to him, it is 

 concerned with the process of the mesoblast formation. He states 



