88 ARTHUR ROBINSON, M.D., AND RICHARD ASSHETON, M.A. 



blastopore are approximated and united, that there should result a 

 mesial axial rod of fused tissue, which cannot in the first instance be 

 resolved into distinct layers. Such a fusion of the blastopore lip was 

 long ago shown to occur in Elasmobranchii by Balfour (1, p. 51), who 

 compared the line of fusion of the lips of the Elasmobranch blastopore 

 to that portion of the blastodermic areas of the Avian ovum to which 

 the term primitive streak had been applied, " The primitive streak 

 represents the linear streak connecting the Elasmobranch embryo with 

 the edge of the blastoderm after it has become removed from its previous 

 peripheral position, as well as the true neurenteric part of the 

 Elasmobranch blastopore" (1. p. 238). ..." That it is in later stages 

 not continued to the edge of the blastoderm, as in Elasmobranchii, is 

 due to its being a rudimentary organ" (1, p. 240). In Balfour's opinion, 

 therefore, the primitive streak represents a portion of the blastoporic 

 opening situated posterior to the neurentric canal, and in conformity 

 with his statement that the anus of Rusconi in the frog is the whole of 

 the blastopore, which becomes gradually contracted, he does not 

 describe a primitive streak in Amphibia. His conclusions, however, 

 are not in accord with those of the upholders of the concrescence theory, 

 who believe that the axial portion of the embryo is formed by the 

 fusion of the lips of an elongated blastopore. The fusion takes place 

 from before backwards, and results in the formation of an axial area, 

 which lies in front of the remains of the blastopore. This area is 

 termed the primitive streak. 



If we examine the anus of Rusconi of Amphibia in the light of 

 Balfour's conclusions concerning the blastopore, and then in that of the 

 concrescence theory, we are forced to believe in the first case that either 

 there is no primitive streak in Amphibians, or that it lies somewhere 

 posterior to the neurenteric canal ; and, further, that it would be 

 completely represented by the fusion of the middle part of the lateral 

 lips of the blastopore in Amblystoma punctatum (39), and in Bombinator 

 according to Goette's description ; or, in the second case, that the anus 

 of Rusconi represents the posterior part of the blastopore, and that the 

 primitive streak is situated in front of it. 



Schwarz (47) and Goette (15) compare the " Prostomaschluss " 

 which lies between the neurenteric canal and anus in Bombinator to 

 the primitive streak of the higher Vertebrates. 



