110 ARTHUR ROBINSON, M.D., AND RICHARD ASSHETON, M.A. 



The Fate of the Primitive Streak in the Frog. 

 In discussing the primitive streak of the frog it will be convenient 

 to consider separately — 



(I) The fate of the ventral moiety. 



(II) The fate of the dorsal moiety. 



Goette (15) has pointed out that in Petromyzon the homologue of 

 the primitive streak must be the whole rim of the blastopore. 

 Apparently, however, he does not extend the same reasoning to the 

 case of the frog. For in that case, according to his description, and 

 to the figure he gives, in which he indicates the position of the primitive 

 streak by a bracket, he does not include the ventral lip of the anus. 



To quote his words, " Das Homologon des Primitifstreifs ist dort 

 die ventrale Halfte des Schwanzes von seiner Spitze bis zum After " 

 (Haut, ventrale Halfte des Schwanzdarms und der Mesoderm platten, 

 Hinterwand des Aftersdarms). 



It is a question how far we ought to speak of the derivatives of the 

 primitive streak being " das Homologon " of the primitive streak. Is 

 it quite correct to say that the adult frog is the homologue of the egg 

 from which it has been developed ? 



However, it is clear that Goette means that the ventral half of the 

 tail is derived from the primitive streak. Here we must again differ 

 from him. 



As the result of our observations we conclude that not only the ventral 

 half of the tail is derived from the primitive streak, but also the dorsal 

 half as well, with the exception of nearly the whole of the shin of the tail. 



This we shall hope to show while considering the fate of the dorsal 

 moiety of the primitive streak. 



"We now proceed to describe the fate of the ventral moiety siibse- 

 quent to the stage corresponding to figs. 23, 15, 16, 17. 



Figs. 20, 21, 22, are camera drawings of sections of the posterior 

 end of a 4^ mm. tadpole, taken at levels corresponding to those of 

 sections 15, 16, and 17 respectively (vide lines 20, 21, 22 in fig. 26, 

 and lines 15, 16, 17 in fig. 23). 



In each case the drawings have been made by camera, and from 

 unstained sections, and the natural appearance as regards tint and 

 shape has been reproduced as nearly as we were able to reproduce it. 



In figs. 20, 21, and 22, there is no trace of a primitive streak, but 



