308 DEVELOPMENT OF ELASMOBRANCH FISHES. 



vertebrates, by the positive embryonic character that their neural 

 and alimentary canals at first communicate posteriorly. The 

 presence or absence of this arrangement depends on the different 

 positions of the embryo in the blastoderm. In Reptiles, Birds 

 and Mammals, the embryo occupies a central position in the 

 blastoderm, and not, as in Pisces and Amphibia, a peripheral 

 one at its edge. We can, in fact, only compare the blastoderm 

 of the Bird and the Elasmobranch, by supposing that in the 

 blastoderm of the Bird there has occurred an abbreviation of the 

 processes, by which the embryo Elasmobranch is eventually 

 placed in the centre of the blastoderm : as a result of this abbre- 

 viation the embryo Bird occupies from the first a central position 

 in the blastoderm 1 . 



The peculiar relations of the blastoderm and embryo, and 

 the resulting relations of the neural and alimentary canal, 

 appear to me to be features of quite as great an importance 

 for classification as the presence or absence of an amnion and 

 allantois. 



General Features of the Hypoblast. 



There are but few points to be noticed with reference to the 

 histology of the hypoblast cells. The cells of the dorsal wall of 

 the alimentary cavity are columnar and form a single row. 

 Those derived from the yolk to form the ventral wall are at first 

 roundish, but subsequently assume a more columnar form. 



1 Vide Note on p. 28 r, also p. 295, and PI. 9, Figs, r and 2, and Comparison, 

 &c., Qy. yi. of Micros. Sci. July, 1875, p. 219. [This Edition, No. VI. p. 125.] 

 These passages give an account of the change of position of the Elasmobranch em- 

 bryo, and the Note on p. 281 contains a speculation about the nature of the primitive 

 streak with its contained primitive groove. I have suggested that the primitive streak 

 is probably to be regarded as a rudiment at the position where the edges of the blas- 

 toderm coalesced to give to the embryos of Birds and Mammals the central position 

 which they occupy. 



If my hypothesis should turn out to be correct, various, now unintelligible, 

 features about the primitive streak would be explained : such as its position behind 

 the embryo, the fusion of the epiblast and mesoblast in it, the groove it contains, &c. 



The possibility of the primitive streak representing the blastopore, as it in fact 

 does according to my hypothesis, ought also to throw light on E. Van Beneden's 

 recent researches on the development of the Mammalian ovum. 



In order clearly to understand the view here expressed, the reader ought to refer to 

 the passages above quoted. 



