FORMATION AND FATE OF THE PRIMITIVE STREAK. 103 



The Primitive Streak of the Frog and other Vertebrates. 



The term primitive streak appears to have been first applied to the 

 dark line which appears in the avian blastoderm at the posterior 

 part of the area pellucida. This line is generally looked upon as the 

 optical expression of a linear thickening and fusion of the blastodermic 

 layers, though Kolliker (31, p. 134) maintains that it is due to 

 proliferation of the epiblast-cells alone. It seems certain, however 

 that it is impossible, during certain periods of the growth of the 

 embryo, to distinguish 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 anteriorly into 

 a perforation, which ultimately becomes the neurenteric 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 ; and 47, pi. xiv, fig. 81, p. 203). 



The lateral margins and posterior extremity of the streak are con- 

 tinuous 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 blasto- 

 derm 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 continuous 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 



