FORMATION AND FATE OF THE PRIMITIVE STREAK. 109 



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

 Bombinator, Triton, and Petroniyzon, as usually described, is not 

 homologous with the primitive streak of Rana temporaria, the Saurop- 

 sida, 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 Elasmobranchs, 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 Amphioxus. With regard to this somewhat 

 anomalous Vertebrate, we can only observe that so far as the observa- 

 tions 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 positive proof, 

 we are still impressed by the fact that the anus is neither the remains 

 of the blastopore, nor is it formed secondarily by reopening of the 

 fused lips of the blastopore, but it appears as a new formation below 

 and in front of the blastoporic opening ; at least Hatschek figures it in 

 that situation and his description is as follows : — " Die Unterbrechung 

 der Communication zwischen Darm, und Medullarrohr erfolgt 

 ungefahr gleichzeitig oder auch etwas sp'ater als der Durchbruch des 

 Afters. Der After bricht ventralwarts von dieser Communications- 

 offnung, die den letzten Rest des Gastrulamundes reprasentirt " (16, 

 p. 79). 



The anus of the frog and many other Vertebrata bears a similar 

 relation to the neurenteric canal, but in no other vertebrate except 

 Amphioxus is the anus a distinctly new formation ; on the contrary, 

 it is very evidently an aperture formed by the reopening of a tempo- 

 rarily closed orifice, or by perforation of the homologue of that orifice. 

 In view of this fact, it seems very probable that future observations 

 will modify Hatschek's results, and remove the contradiction which at 

 present exists between Amphioxus and other Vertebrata. If, how- 

 ever, this proves not to be the case, it will then have to be decided 

 whether the condition which is found in Amphioxus, or that which is 

 so general among the other Vertebrata, is the more primitive. 



