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\ 



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

 the air, acquires a yellower hue. The true germinal disc ap- 

 pears as a circular spot of a bright orange colour, and is, accord- 

 ing to Leydig's measurements, IJm. in diameter. Its colour 

 renders it very conspicuous, a feature which is further increased 

 by its being surrounded by a narrow dark line (PI. I. fig. 2), 

 the indication 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 gra- 

 dations into the yolk. As was mentioned in my preliminary 

 paper {loc. cit.), and as Leydig (loc. cit.) had ^^efore noticed, the 

 germinal disc is always situated at the pole of the yolk which 

 is near the rounded end of the Pristiurus e^g. 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 Qgg examined, exhibited two furrows which 



1 Rochen und Ilaie. 



