480 ARTHUR ROBINSON AND RICHARD ASSHETON. 



of the layers in the lips of the blastopore and by concrescence 

 of the lips of the blastopore, either from before backwards as in 

 Triton (Goette, 15), or by fusion of the lateral lips of the 

 blastopore between the neurenteric canal and the anus, arriv- 

 ing at its completion on the obliteration of the neurenteric 

 canal, as in Bombinator (15) and Amblystoma punctatum 

 (39); or it may be formed, as we have already shown in Rana 

 temporaria, by fusion of the lips of the blastopore from behind 

 forwards. 



In Teleosteans and Cyclostomes it is formed by fusion of 

 the blastoporic lips from before backwards, also whilst the 

 archenterou is being formed and extended, and in Elasmo- 

 branchs by fusion of the lateral lip of the blastopore behind 

 the neurenteric canal. 



In the Sauropsida and Mammalia we have no definite proof 

 of 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 

 Ban a temporaria. It is an area of fusion of the germinal 

 layers which afterwards becomes perforated by the formation 

 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 lieterochronous displacement asso- 

 ciated 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 blas- 

 todermic 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 



