92 THE NOTOCHORD. 



teriorly. 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 sup- 

 posing that in the blastoderm of the Bird there has occurred 

 an abbreviation of the processes, by which the embryo Elasmo- 

 branch is eventually placed in the centre of the blastoderm : as 

 a result of this abbreviation the embryo Bird occupies from 

 the first a central position in the blastoderm\ 



The peculiar relations of the blastoderm and embr3^o, 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 hypohlast. 



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. 



The Notochord. 



One of the most interesting features in the Elasmobranch 

 development is the formation of the notochord from the hypo- 



1 Vide Note on p. 68, also p. 81, and PI. viii. Fig. 1 and 2, and Comparison, 

 &c., Qy. Jour, of Mic7'os. ScL July, 1875, p. 219. These passages give an 

 account of the cliauge of position of the Elasmobranch embryo, and the Note on 

 p. 08 contains a speculation about the nature of the primitive streak with its 

 contained primitive groove. I have suggested that the primitive streak is pro- 

 bably to be regarded as a rudiment at the position where the edges of the 

 blastoderm 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 

 lieneden'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. 



