FATE OF BLASTOPORE, ETC., IN CHELOXIA. 19 



following :ire some of these instances : — Strahl (81), Taf. VI, Figs. 

 2 & 2a (like Fig. 2 of the present contribution) ; Mitsukuri & 

 IsHIIvA^YA, Contrib. I, Fig. 4a; Mehnert ('91), Tafel XA^III, Fig. 

 17a, 17b, 18a (Cdl. Knt); Clarke ('91), PL XI, Figs. 27 & 28 (like 

 Fig. 17 of the present contribution?). Kupffer ('92) gives what 

 appears to be a similar stage for tlie sparrow in his Taf. VIII, Fig. 

 6. It would no doubt be easy to add other instances, but these will 

 be sufficient to show that such a stage has very frequently been 

 noticed. 



I have ah'eady quoted the views of two of the most recent writers 

 on reptilian development as to the fate of this yolk-plug, xiccord- 

 ing to Mehnert, it becomes eventually fused with the posterior ends 

 of the medullary folds and f ^rtus, when the tail is folded, its extreme 

 posterior end. According to Will, the blastopore lips gradually 

 extend themselves backwards, until they meet behind, and entirely 

 circumscribe, the cell-mass in cpiestion, which then becomes pushed in 

 and disappears from the sur lace. All this takes place presumably before 

 the upheaval of the medullary folds. 



In Contrih. IV, I accepted Will's views as probably correct in the 

 main, but I shall now proceed to show that the true course of develop- 

 ment is ditferent from either of the above views. In doing so, I find 

 it most convenient, from the stage of Figs. 2, 9, and 18 onward, to 

 separate Chelonia and Ckmniijs from Trionijx, as the former two display, 

 in my opinion, the more primitive course of development, which 

 is considerably obscured in Trioniß:. 



Laier Stages. 



I take up first 



Chelonia caovana. 

 I would ask the reader to study carefully Figs. 2-8a which I 



