FATE OF BLASTOPORE, ETC., IX CHELOXIA. 97 



Elasmobranchii. That part marked Z'.S' in Fig. 23 of Robixsox and 

 AsSHETOX corresponds, on the wliole, with Woodcut X, C of this 

 article. So far, well and good. But it is insisted on lîv 0. Hertwig 

 ('92, p. 452 et. seq.) that the anus in Amphibia lies at tlie posterior end 

 of the blastopore. If this is i-eally so, then what Ijecomes in Am- 

 phibia of the pnrt corresponding to the non-embryonic portion of the 

 primitive streak situated behind the anus in Chelonia and Elasmo- 

 branchii — ef (J, Woodcuts XII, XIII, and XIV? Oi-, is the amphi- 

 bian anus an altogether different structure from the chelonian or 

 selachian, as in the latter two groups the anus is not plainly at the 

 posterior end of the primitive streak? It is stated by Robixsox and 

 AssHETOx that there are twenty sections of the primitive streak 

 behind the anus in the liana embryo figured in their Fig. 23. Sup- 

 posing the anus to be homologous in Chelonia and Elasmobranchii 

 on the one hand and Amphibia on the other, does that short stretch 

 represented by twenty sections in the latter group represent the 

 coalesced edge and the edge around the yolk-ljlastopore (ef(j. Wood- 

 cuts XII-XIV) of the meroblastic egg ? I leave these questions 

 open, for I think. M'e liave at present no means of answering them 

 satisfoctorily. At any rate, the above discussi(3n makes it certain 

 that the Rusconian anus in Amphibia is not homologous with the 

 volk-blastopore of Elasmobranchii: the former includes a great deal 

 more than the latter, lor it gives rise to the neurenteric canal, the 

 anus, and the primitive streak between the two. The Rusconian anus 

 of the Amphibia corresponds rather, though not exactly, to the 

 problematical cell-mass of Chelonia in such a stage as Fig. I, Rl. I., 

 but not to the latter when it has moved Ijackwards and represents 

 only the yolk-blastopore (Fig. 8). Those who share Hertwig's 

 opinion that there is no such thing as the yolk-blastopore in Elasmo- 

 branchii f'92, p. :/3^6' and Lelirhueli, 4te Auß., p. 134) may consider 



