FORMATION AND FATE OF THE PRIMITIVE STREAK. 481 



which is perforated posteriorly by the anus, and may be per- 

 forated, more or less completely, anteriorly by the neurenteric 

 canal, — then we are bound to admit that this region and the 

 corresponding areas in mammalian and reptilian blastoderms 

 are the homologues of the primitive streak of Rana tempo- 

 raria; and conversely we are forced to conclude that, as the 

 primitive streak ofRana temporaria is formed by the linear 

 fusion of the undifferentiated area in the lips of the blastopore, 

 the primitive streak of the Sauropsida and Mammalia is homo- 

 logous with the fused lip of the blastopore of Rana. 



Strictly speaking, therefore, the typical primitive streak is 

 an area which extends antero-posteriorly from the point at 

 which the fusion of layers commences, in front of the anterior 

 lip of the blastopore (fig. 10, Z), to the point at which the 

 fusion of layers terminates behind the posterior lip of the 

 blastopore (fig. 10, Z') ; and laterally from the termination of 

 the fusion on the left lip of the blastopore (fig. 10, ZZ'), to 

 the termination of the fusion of the layers in the right lip of 

 the blastopore (fig. 10, ZZ). 



If this is the case, then it is evident that the primitive streak 

 of Bombinator, Triton, and Petromyzon, as usually described, 

 is not homologous with the primitive streak of Rana tempo- 

 raria, the Sauropsida and Mammals, but only with that 

 portion of it which lies in front of the anus. And it is also 

 evident that the primitive streak of Teleosteans and Elasmo- 

 brauchs, after the complete closure of the blastopore, is the 

 exact homologue of the typical primitive streak as formed in 

 birds. 



With regard to the Ganoids, it is only possible to say that 

 they seem to closely resemble the Teleosteans. It will be 

 noticed that we have made no reference to Araphioxus. With 

 regard to this somewhat anomalous Vertebrate, we can only 

 observe that so far as the observations of Hatschek (16) and 

 Kowalevsky (32) go, there is no primitive streak formed, 

 unless we accept in full the concrescence theory, and look 

 upon the dorsal axial line as its representative ; but even if 

 we adopt this theoretical conception, of which there is no 



VOL. XXXII, PART IV. — NEW SER. K K 



