FORMATION OF GERMINAL LAYERS IN CHELONIA. 239 



out through other classes of Vertebrata. We need hardly say that 

 our investigations most completely bear out Balfour's and Hertwig's 

 view. In fact, the agreement between the development of Amphioxus 

 and Amphibia on one side, and of Reptilia on the other, as shown by 

 our work, is as complete as could be desired, when we make due 

 allowance for the fact that on one side is a holoblastic and on the 

 other a meroblastic egg. Let us examine more in detail. 



When we compare our fig. 16 with Hertwig's fig. 4 (Taf. ii, No. 6) 

 of Triton, we are at once struck with the close similarity between 

 the two, allowing for the fact that the latter represents a whole egg, 

 and the former only a small part of it. There is in both a passage 

 connecting the cavity which becomes the future alimentary canal 

 with the exterior. This is, according to Hertwig's nomenclature, 

 "die enger Theil der Darmhöhle (tZ/t.)," according to ours "the 

 blastoporic passage." At the dorsal lip of this passage the ectoblast 

 in both is reflected, and becomes continuous with the chorda-ento- 

 blast. In the region in front of the passage the embryo consists of 

 only the ectoblast and entoblast. In both there is the yolk-plug 

 liehind the passage, and contiguous with it the two primary layers 

 are fused, and from the fused point there stretches backward an 

 unpaired mesoblastic mass. Hertwig's fig. 11, Taf. v, and figs. 7 

 and 10, Taf. vi, of Rana, are essentially alike. 



Hertwig's fig. 17 (Taf. iv) is the frontal section throiîgh the line 

 a — h of fig. 4, Taf. ii. It passes through the beginning of the 

 unpaired mass of mesoblast. It presents an appearance very similar 

 to our fig. 8 of the corresponding region. The ectoblast is pro- 

 liferating in the median line, and giving cells to the mesoblast. 

 In our figure the entoblast and mesoblast are separate, but we 

 have shown already that they become ecmtiniious further forward. 

 Hence exactly the same relations hold in this region in Triton and 



