ON THE PROCESS OF GASTRULATION IN CHELONIA. 259 



to go over the writings of these authors and point out wherein we 

 agree or differ. The reader acquainted with the hterature will be 

 able to do this for himself. The points which I want specially to 

 emphasize are however as follows : — 



1. The pklmitive; plate or knob is raw-material left at the 

 centre of tue blastoderm, by means of which certain palingenetic 

 processes are gone tlirough. 



'J. The INVAGINATION-CAVITY is the APCHENTERON, and givcs rise 

 to the alimentary canal and the organs derived from it exclusive of 

 the proctodti3um and the stomodseum. 



o. The YOLK-SAC must be regarded as a ventral appendage or 

 diverticulum of the AiiCHENTERON in which nutritive matter is stored 

 in solid or liquid form. 



4. ÜAving to the enormous size of the yolk-sac, it and the 

 AKCHEXTEKON are formed separately from each other, and come only 

 secondarily into connection. 



Having considered the chelonian egg in its relations with that 

 of Ichthyopsida let us now see how it compares with the avian or 

 mammalian egg. 



If the process by which the blastopore in Chelonia has assumed 



a horse-shoe shape (W oodcut V. A) continues on after the state A 



is reached, as actually happens in Amphibia (See Figs. 

 Woodcut V. 

 ^^.^^ 18 & ID, Ao. 10), the lateral lips will coalesce* and 



I there Avill result the primitive streak of the avian egg 

 (]>). In cases where the lips have not quite coalesced, 

 we .should expect to find the ^^olk-plug sticking out 



* I am gratified to find this verified within the group of Re'ptilia. Will (No. 21) has found 

 in Gecko that the lips of the blastopore approach each other very closely and form a primitive 

 streak. 



