458 ARTHUR ROBINSON AND RICHARD ASSHETON. 



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 poste- 

 rior to the neurenteric canal, and in conformity with his state- 

 ment 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 neu- 

 renteric canal ; and, further, that it would be completely repre- 

 sented by the fusion of the middle part of the lateral lips of 

 the blastopore in Arablystoma 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 '' Prostoma- 

 schluss" which lies between the neurenteric canal and anus in 

 Bombinator to the primitive streak of the higher Vertebrates. 



Morgan (39), Alice Johnson (27), and Schultze (45) mention 

 a primitive streak in front of the blastopore. Morgan gives no 



