108 ARTHUR ROBINSON, M.D., AND RTCHARD ASSHETON, M.A. 



primary blastoporic opening, for the aperture so named by van Beneden 

 in the rabbit (4), and by Selenka in the opossum (50), is not yet 

 definitely located ; but the primitive streak in these Vertebrates is 

 readily comparable with the secondary condition of the Amphibian 

 blastopore as formed in Rana temporaria. It is an area of fusion of 

 the germinal layers which afterwards becomes perforated by the for- 

 mation of the anus. The appearance of the fused area at a compara- 

 tively late stage in reptiles, birds and mammals — that is, after the 

 archenteron is well established (if we except Kupffer's views on the 

 function of the archenteron of reptiles, according to which the primitive 

 gut is produced by invagination) — throws but little difficulty in the 

 way of the comparison, for it is most probably a mere heterochronous 

 displacement associated with the precocious segregation of the 

 hypoblast in the higher Vertebrates. 



Therefore, if we use the term primitive streak in the sense in which 

 it is used in the description of the avian blastoderm — that is, as a 

 term which signifies an area of fusion of the blastodermic layers which 

 is continuous laterally and posteriorly with the separated epiblast, 

 mesoblast, and hypoblast, and is in front continuous with the neural 

 plate and chorda, and which is perforated posteriorly by the anus, 

 and may be perforated, 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 temporaria ; and 

 conversely we are forced to conclude that as the primitive streak of 

 Rana 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 homologous 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). 



