232 RICHAED ASSHETON. 



cavity anteriorly by means of a splitting, the result of the 

 primary area of activity, and posteriorly by means of a growth 

 backwards of the dorsal lip of the blastopore, the result of the 

 secondary area of activity, comparable to the corresponding 

 parts in the rabbit, formed previous to the eighth day, and 

 upon and subsequently to the eighth day respectively. In the 

 rabbit and in Amphioxus the lining of the archenteron of the 

 primary area is completed before the secondary area of pro- 

 liferation has become established, but in the frog afterwards ; 

 and so the linings of both parts of the gut cavity are formed 

 together. 



Many actual experiments and observations have been made 

 upon the eggs of the frog with the object of demonstrating the 

 mode of formation of the blastopore, and the relative position 

 of the blastopore when it has a completed margin to the 

 originally black and white poles of the unsegmented ovum. 

 Such attempts have been made with varying results by Roux, 

 Schultze, Hertwig, Morgan, and Ume Tsuda, and although the 

 experiments described are in many cases contradictory, yet 

 there seems to be no doubt that the dorsal lip of the blastopore 

 does overgrow a portion of the whiter side of the embryo prior 

 to the completion of blastoporic lip ventrally. 



I have myself made similar experiments in repetition of 

 E,oux, and I am quite convinced that this overgrowth does 

 occur to a certain extent, but I am equally sure that it is 

 incorrect to assert that the neural plate is formed entirely 

 upon the lower (white) pole of the ovum. 



The dorsal lip overgrows the white segments, at 

 any rate apparently, but so do the lateral and 

 ventral lips as they are formed. It is only because 

 the dorsal lip is formed first that this part seems 

 to overgrow the white i)ole to so large an extent. 



The overgrowth is a part of the same process which pro- 

 duces the lengthening of the embryo. If the whole blastoporic 

 lip could in the frog be formed at once the embryo would, I 

 suspect, change rapidly from a sphere to an oval, as does the 

 embryo of Amphioxus (v. Hatschek, figs. 30 and 34). 



