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K. MITSUKUEI AND C. ISHIKAWA. 



earlier than fig. 1 . We will try, however, to reason back from our 

 earliest stages and to deduce what processes have given rise to such 

 a form. Of course, such à priori reasoning is liable to mistakes, 

 and we offer the following remarks merely as suggestions which need 

 verification by future investigations. If the blastoporic passage 

 really commences as an epiblastic invagination, it seems to us that 

 Kupffer is quite right in considering the invaginated sac as the 

 gastrula cavity much reduced in size (No. 5, p. 2). But apart from 

 the inherent improbability that the bottom of the archenteron should 

 afterward give way and the archenteron should become connected 

 with some cavity beyond itself, we think we have another sufficient 

 reason in rejecting the view of an epiblastic invagination in this fact 

 that directly behind the passage, when it is established, there is an 

 area which is not covered by the ectoblast, i. e. the yolk-plug. We 

 think then that what really takes place must be very much as 

 Weldon and Strahl describe it, for these two writers differ after all, 

 when we leave out minor points, only in this, that the former thinks 

 the passage arises at the front end, and the latter at the middle 

 of the primitive streak. Our views, then, on these earliest stages are 

 as follows : 



At the end of the segmentation the blastoderm becomes divided 

 into two primary layers, the ectoblast above consisting of columnar 

 cells, and the entoblast below consisting of irregularly shaped cells 

 without any definite arrangement. At the region of the future 

 blastopore and primitive streak, this process of diff'erentiation is 

 somewhat modified from what takes place elsewhere. When the 

 differentiation of the ectoblast has proceeded backward and come to 

 the future dorsal lip of the blastopore, it does not extend further in 

 the median line over the blastodermic surfiice, but becomes reflected 

 downward and continuous Avith the axial strip of the lower layer 



