FORMATION AND FATE OF THE PEIMITIVE STREAK. 475 



during certain periods of the growth of the embryo, to distin- 

 guish the three germinal layers from each other in the area of 

 the primitive streak (Duval, 10, p. 181). 



After a time differentiation proceeds in the streak ; a groove 

 appears on its superficial surface, and this is deepened ante- 

 riorly into a perforation, which ultimately becomes the ueuren- 

 teric canal (47). The anterior wall of this perforation is 

 formed by a mass of cells in which the epiblast and hypoblast 

 are united (12). In the posterior portion of the streak the 

 anal membrane is developed by the gradual thickening and 

 apposition of the hypoblast and epiblast (13, pi. x, figs. 4 and 

 5, p. 299 i and 47, pi. xiv, fig. 81, p. 203). 



The lateral margins and posterior extremity of the streak 

 are continuous with the mesoblast, which appears to grow out 

 from them into the surrounding area. 



The anterior end of the streak is continuous with the chorda 

 ventrally, and the central part of the neural plate dorsally. 

 In the region surrounding the primitive streak the three layers 

 of the blastoderm are distinct from each other, except in front 

 of the anterior extremity of the streak, where the so-called 

 " Kopffortsatz," the first rudiment of the chorda, is still con- 

 tinuous with the entoderm. The connection between the 

 hypoblast and the mesoblast, which exists throughout the 

 whole length of the streak, is first dissolved posteriorly, where 

 for a certain period the epiblast and mesoblast are fused, but 

 the hypoblast forms a distinct layer. 



Between the primitive streak of a bird and the frog there 

 are resemblances and differences of importance. In the frog 

 the primitive streak is formed by a concrescence of the lips of 

 the blastopore, which proceeds from behind forwards, and 

 which is only completed on the obliteration of the neurenteric 

 canal. In the bird the primitive streak is formed from before 

 backwards according to Duval (10) and Schwarz (47), from 

 behind forwards according to Balfour and Deighton (2) and 

 KoUer (30) ; and the appearance is due apparently to thicken- 

 ing and fusion of the two primary layers — Duval (10), Balfour 

 and Deighton (2). 



