ON THE PROCESS OF GASTRULATION IN CHELONTA. 237 



probably not complete. But as to the above ])oint there is not room 

 lor mneli doubt. The specimens are very much like Fig. 1 of Will 

 (No. 19) with one exception, stated above, viz : that the epiblast of 

 the shield is continuous with the primitive plate and not separate 

 as in Will's figure. One peculiarity of this stage is that both the 

 anterior and the posterior wall of the invagination shows faintly the 

 columnar arrangement as seen in Will's figure. Later on, this feature 

 is confined to the anterior or dorsal wall (Figs. 9, 13, 14). After going 

 straight downwards some distance the invagination cavity takes a for- 

 ward horizontal direction and reaches the conditiim shown in Fig. 1>. 

 At the anterior lip of the blastopore, the columnar cells are recognizable 

 very early, and the epiblast is here reflected downwards to l)ecoine con- 

 tinuous with the anterior or dorsal wall of the invagination. In that 

 part of the primitive ])late placed behind the invagination the cell- 

 mass remains undifierentiated for a long time, there l)eing later estab- 

 lished in this place the rudimentary yolk-plug, as was minutely describ- 

 ed in the joint paper of Ishikawa and myself* on the germinal layers of 

 Trionyx. Robinson and Assheton (No. 10) object to our idea of 

 considering the structure in question as the yolk-plug. In the course 

 of tliis paper, I hope to show that the presence of the yolk-plug at 

 this place is an important feature in homologising the gastrulation of 

 the Sauropsida with that of Amphibia. I may add that several 

 authors, as Van I^eneden (Xo. 13), Wenckebach (No. 15), and Will 

 (Nos. 18 and 21) recognise the yolk-])lug in this place. 



I shall next describe how the invagination-cavity, as described 

 above, comes to open below ;uid l)ecomes united with the large subger- 

 minal cavity in the yolk. This process has, ,so far as I am aware, 



* Coatribiition I. I shall refer to the papers in the present series of Contrilnitions by their 

 nuuiliers in order of publication. See the list at the end of the present article. 



