DEVELOPMENT OF VERTEBRATES. 133 



never becomes either the one or the other of these openings, it 

 is, I think, possible to account for its corresponding in position 

 with the mouth in some cases or the anus in others. 



That it would soon come to correspond either with the 

 mouth or anus (probably with the earliest formed of these in 

 the embryo), wherever it was primitively situated, follows from 

 the great simplification which would be effected by its doing so. 

 This simplification consists in the greater facility with which the 

 fresh opening of either mouth or anus could be made where the 

 epiblast and hypoblast were in continuity than elsewhere. Even 

 a change of correspondence from the position of the mouth to 

 that of the anus or vice versa could occur. The mode in which 

 this might happen is exemplified by the case of the Selachians. 

 I pointed out in the course of this paper how the final point of 

 envelopment of the yolk became altered in Selachians so as to 

 cease to correspond with the anus of Rusconi ; in other words, 

 how the position of the blastopore became changed. In such a 

 case, if the yolk material again became diminished, the blasto- 

 pore would correspond in position with neither mouth nor anus, 

 and the causes which made it correspond in position with the 

 anus before, would again operate, and make it correspond in 

 position perhaps with the mouth. Thus the blastopore might 

 absolutely cease to correspond in position with the anus and 

 come to correspond in position with the mouth. 



It is hardly possible to help believing that the blastopore 

 primitively represented a mouth. It may perhaps have lost 

 this function owing to an increase of food yolk in the ovum 

 preventing its being possible for the blastopore to develop 

 directly into a mouth, and necessitating the formation of a 

 fresh mouth. If such were the case, there would be no reason 

 why the blastopore should ever again serve functionally as a 

 mouth in the descendants of the animal which developed this 

 fresh mouth. 



