470 ARTHUR ROBINSON AND RICHARD ASSHETON. 



these structures are the homologue of those parts in the chick 

 embryo to which these names were originally given. 



A section taken dorsal to the blastopore, and dorsal to the 

 fusion of layers at the dorsal lip of the blastopore, is seen in 

 fig. 12. 



Here the epiblast is seen to be quite distinct from the meso- 

 blast. There is a distinct groove, the neural groove, very 

 different in character from that of the groove ventral to the 

 blastopore, or primitive groove, while the epiblast is thickened 

 on either side to form the neural plate. The level of the re- 

 spective sections is marked in fig. 11 by the lines numbered 

 12 and 13. 



Before discussing the relations between the structures, the 

 formation of which we have been following in the last two or three 

 pages, and which we have named primitive streak and primi- 

 tive groove, and the primitive streak and primitive groove of the 

 chick, we will follow its history a little further, until the stage 

 at which we may say it has attained its greatest development. 



We must, therefore, describe the condition of the primitive 

 streak of a frog embryo of about 2| mm. in length ; that is to 

 say, an embryo in which the neural folds are on the point of 

 meeting, or have just met, as shown in fig. 23. 



Primitive Streak of the Frog at the Time of the 

 Closure of the Neural Folds. 



Fig. 23 is the surface view of the posterior end of a 2^ mm. 

 frog embryo, in which the neural folds have just met. 



It was drawn partly from a preserved specimen, and partly 

 from a model, as described on page 453. 



The line of junction of neural folds (/) is marked by a deep 

 groove ending posteriorly in the blastopore. 



The blastoporic opening, as seen from the surface, is no 

 longer circular, but is more or less lozenge-shaped, due to the 

 folding its dorsal lips have undergone (as will be described 

 later). 



