ON THE PATE OP THE BLASTOPOBB IN THE NEWT. 173 



In Petromyzon, an invagination takes place. The blasto- 

 pore remains open for a long time, though not permanently. 

 The medullary canal is formed first as a solid cord, which 

 becomes continuous with the hypoblast at the lip of the 

 blastopore, thus forming the rudiment of a neurenteric canal. 



In Acipenser, the invagination blastopore is converted into a 

 neurenteric canal. 



In Lepidosteus, there is no open blastopore of the ordinary 

 kind, formed by means of an invagination, but Professor 

 Balfour says : " In the region of the tail, the axial part of the 

 hypoblast, the notochord and the neural cord fuse together, 

 and the fused part so formed is the homologue of the neuren- 

 teric canal of other types. Quite at the hinder end of the 

 embryo, the mesoblastic plates cease to be separable from the 

 axial structures between them " {' Comp. Embryology, vol. ii, 

 p. 93). This arrangement seems to be comparable with the 

 primitive streak and neurenteric canal at its front end, such as 

 is found in the higher Chordata. 



In Amphibians generally, the invagination blastopore gives 

 rise to a neurenteric canal. In the Newt, however, the in- 

 vagination blastopore becomes the anus. A primitive streak 

 extends along the dorsal surface in front of the blastopore, and 

 I believe that there is no neurenteric canal. The primitive 

 groove, which extends in front of the medullary folds, has a 

 deep pit at its anterior end. In Amblystoma also, as men- 

 tioned above, the blastopore probably becomes the anus. 



In Reptiles, there is an invagination blastopore, which 

 becomes a neurenteric canal. Behind this point there is a 

 primitive streak. The anus is formed along the line of the 

 primitive streak, which extends at least as far forwards as the 

 opening of the allantois into the alimentary canal in the 

 Lizard,^ and probably in all types having an allantois. StrahP 

 states that in the Lizard, the invagination begins in the middle 

 of the primitive streak, near, but not at, its front end. By the 



> W. F. R. Weldon, "Note on the Early Development of Lacerta 

 mural is,' this Journal, January, 1883. 

 ' H. Strahl, loc. cit. 



