FORMATION AND FATE OF THE PRIMITIVE STREAK. 483 



To quote his words, "Das Homologon des Primitifstreifs 

 ist dat 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 deriva- 

 tives of the primitive streak being " das Homologon '^ of the 

 primitive streak. Is it quite correct to say that the adult 

 frog is the horaologue 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 dififer 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 skin 

 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 

 subsequent 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 correspond- 

 ing 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 all three germinal layers are now distinct and 

 separate. In fig. 22 the mesoblast ME may be said to be still 

 fused with the epiblast EF'" , but it is, nevertheless, quite 

 distinct in character of cell, and in degree of pigmentation. 

 In the two other sections, figs. 21 and 20, the mesoblast has 

 become entirely separated from the epiblast. 



By comparing these three sections with the sections from 



