DEVELOPMENT OF ELASMOBRANCH FISHES. 11 



crossed each other at right angles in the centre of the disc, but 

 neither of which reached its edge. These furrows accordingly 

 divided the disc into four segments, completely separated from 

 each other at the centre of the disc, but united near its cir- 

 cumference. 



I made sections, though not very satisfactorily, of this 

 germinal disc. The sections shewed that the disc was composed 

 of a protoplasmic basis, in which were imbedded innumerable 

 minute spherical yolk-globules so closely packed as to constitute 

 nearly the whole mass of the germinal disc. 



In passing from the coarsest yolk-spheres to the fine sphe- 

 rules of the germinal disc, three bands of different-sized yolk- 

 particles have to be traversed. These bands graduate into one 

 another and are without sharp lines of demarcation. The outer 

 of the three is composed of the largest-sized yolk-spherules 

 which constitute the greater part of the ovum. The middle band 

 forms a concentric layer around the germinal disc, and is com- 

 posed of yolk-spheres considerably smaller than those outside it. 

 Where it cuts the surface it forms the zone of lighter colour 

 immediately surrounding the germinal disc. The innermost 

 band is formed by the germinal disc itself and is composed of 

 spherules of the smallest size. These features are shewn in 

 PI. I. fig. 6, which is the section of a germinal disc with twenty- 

 one segments ; in it however the outermost band of spherules is 

 not present. 



From this description it is clear, as has already been men- 

 tioned in the description of the ripe unimpregnated ovum, that 

 the germinal disc is not to be looked upon as a body entirely 

 distinct from the remainder of the ovum, but merely as a part 

 of the ovum in which the protoplasm is more concentrated and 

 the yolk-spherules smaller than elsewhere. Sections shew that 

 the farrows visible on the surface end below, as indeed they do 

 on the surface, before they reach the external limit of the finely 

 granular matter of the germinal disc. There are therefore at 

 this stage no distinct segments: the otherwise intact germinal 

 disc is merely grooved by two furrows. 



I failed to observe any nuclei in the germinal disc just 

 described, but it by no means follows that they were not 

 present. 



