FORMATION AND FATE OF THE PRIMITIVE STREAK. 99 



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

 ayers at the dorsal lip of the blastopore, is seen in fig. 12. 



Here the epiblast is seen to be cpiite distinct from the mesoblast. 

 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 respective sections is marked 

 in fig. 11 by the lines numbered 12 and 13. 



Before discussing the relations between the structures, the forma- 

 tion of which we have been following in the last two or three pages, 

 and which we have named primitive streak and primitive 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 83. 



The line of junction of neural folds (J) 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). 



The lengthening of the embryo has removed from the blastopore all 

 trace of the "yolk-plug." 



Kunning ventrally from the blastopore is the primitive groove, a 

 rather narrow but now sharply defined groove near the ventral end, 

 which is suddenly deepened into a pit. 



Beyond the pit the groove continues a short distance. This pit is the 

 commencement of the anus, which will shortly perforate at this spot. 



