CHAPTER II. 

 THE SEGMENTATION. 



I HAVE not been fortunate enough to obtain an absolutely 

 complete series of eggs during segmentation. 



In the cases of Pristiurus and Scyllium only have I had any 

 considerable number of eggs in this condition, though one or 

 two eggs of Raja in which the process was not completed have 

 come into my hands. 



In the youngest impregnated Pristiurus eggs, which I have 

 obtained, the germinal disc was already divided into four seg- 

 ments. 



The external appearance of the blastoderm, which remains 

 nearly constant during segmentation, has been already well 

 described by Leydig 1 . 



The yolk has a pale greenish tinge which, on exposure to the 

 air, acquires a yellower hue. The true germinal disc appears as 

 a circular spot of a bright orange colour, and is, according to 

 Leydig's measurements, lim. in diameter. Its colour renders it 

 very conspicuous, a feature which is further increased by its 

 being surrounded by a narrow dark line (PI. 6, fig. 2), the indica- 

 tion of a shallow groove. Surrounding this line is a concentric 

 space which is lighter in colour than the remainder of the yolk, 

 but whose outer border passes by insensible gradations into the 

 yolk. As was mentioned in my preliminary paper (loc. cif.\ and 

 as Leydig (loc. cit.} had before noticed, the germinal disc is 

 always situated at the pole of the yolk which is near the rounded 

 end of the Pristiurus egg. It occupies a corresponding position 

 in the eggs of both species of Scyllium (stellare and canicula) 

 near the narrower end of the egg to which the shorter pair of 

 strings is attached. The germinal disc in the youngest egg 



1 tioi/'ien nnd Haic. 



