ON THE PATE OP THE BLASTOPORE IN THE NEWT. 176 



enteric canal. Sometimes the blastopore merely consists of a 

 primitive streak with no opening at all. 



In all cases, except the Newt, the opening is restricted to 

 the embryonic stages, and may be described as an embryonic 

 structure. As to the variations, we can only say, for want of 

 a more definite reason, that they are for purposes of embryonic 

 convenience. 



It is obvious, from all the facts adduced, that the original 

 Vertebrate blastopore was elongated, but its present condition 

 shows that great changes have taken place, since, even in the 

 embryo, part or parts of the opening have been obliterated, 

 these parts varying in different embryos. It is obvious, too, 

 that the anus, at any rate, is derived from the original blasto- 

 pore, and therefore is probably not an entirely new formation, 

 acquired within the group, but is homologous with the anus of 

 the primitive ancestral form. 



My results show also that the primitive streak (i. e. blasto- 

 pore) extends much further forwards than was supposed. In 

 fact, the pit found at the front end of the primitive groove in 

 the Newt corresponds in position more or less with the future 

 mouth as has been remarked. This points to the probability 

 of a connection between the blastopore and mouth, and so 

 supports Mr. Sedgwick's^ view that the blastopore of the 

 Chordata was an elongated dorsal slit, the ends of which gave 

 rise to the mouth and anus. 



In Peripatus,^ too, it is known that the blastopore is an 

 elongated structure, the middle part of which closes, while the 

 ends become respectively the mouth and anus of the adult. 



The fusion of the embryonic layers is most distinct at the 

 hind end of the embryo. I believe that it exists also at the 

 front end of the primitive groove in the Newt. In the middle 

 region of the body its existence is doubtful, but the fact that 

 the primitive groove extends along the dorsal surface from the 



* A. Sedgwick, loc. cit. 



° F. M. Balfour, "Anatomy and Development of Peripatus capensis," 

 this Journal, April, 1883. 



