GROWTH IN LENGTH OF THE FROG EMBRYO. 231 



the secondary growing point becomes a functionally active area 

 in Araphioxus. It is possible for it to become established as 

 soon as a blastoporic lip is formed^ and not before, because a 

 characteristic feature of this secondary area of proliferation is 

 that it should produce cellular units to all existing cellular 

 layers. 



I have in a previous paper tried to locate this line of demar- 

 cation in the rabbit. The moment of origin of the secondary 

 area of proliferation in the rabbit is fairly well marked. Of 

 Amphioxus I cannot speak. Can we find it in the frog ? The 

 frog is by no means so simple as the rabbit, but is more 

 amenable to experiment than is Amphioxus. 



The frog differs in one respect from Amphioxus, which is of 

 importance in reference to the question now under discussion. 

 In Amphioxus the blastoporic lip is formed at the same 

 moment apparently at all points of its circumference. In 

 the frog it is formed at one point first, namely, at the future 

 dorsal region, and many hours elapse before the lip is formed 

 ventrally. Hence it is possible for the secondary 

 area of proliferation to become established much 

 sooner dorsally than ventrally. 



In my account of the frog given above, the production of the 

 split forming the primitive archenteron, which I believe to 

 represent the process of gastrulation of Amphioxus, although 

 no invagination occurs, is to be considered, like the invagina- 

 tion process in Amphioxus, as the result of the primary area 

 of proliferation, and of itself would tend to the production of a 

 radial symmetry. The very moment this split begins, a portion 

 of the blastoporic lip is thereby formed. 



If my supposition is right, that the secondary area of pro- 

 liferation may be established as soon as there is a mass of 

 cellular tissue in connection with all the primary layers, it is 

 clear that possibly the secondary area of proliferation may 

 start immediately upon the formation of the dorsal lip of the 

 blastopore, and not delay until the whole blastoporic rim is 

 completed. 



Accordingly, on this view, there will be an extension of gut 



