In passing from the coarsest yolk-spheres to the fine spherules 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 composed 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 Pl. 6, 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 mentioned 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 furrows 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. In the next youngest of the eggs[80] examined the germinal disc was already divided into twenty-one segments. When viewed from the surface (Pl. 6, fig. 3), the segments appeared divided into two distinct groups--an inner group of eleven smaller segments, and an outer group of segments surrounding the former. The segments of both the inner and the outer group were very irregular in shape and varied considerably in size. The amount of irregularity is far from constant and many germinal discs are more regular than the one figured. Footnote 80: The germinal disc figured was from the egg of a Scyllium stellare and not Pristiurus, but I have also sections of a Pristiurus egg of the same age, which do not differ materially from the Scyllium sections. In this case the situation of the germinal disc and its relations to the yolk were precisely the same as in the earlier stage. In sections of this germinal disc (Pl. 6, fig. 6), the groove which separates it from the yolk is well marked on one side, but hardly visible at the other extremity of the section. Passing from the external features of this stage to those which are displayed by sections, the striking point to be noticed is the persisting continuity of the segments, marked out on the surface, with the floor of the germinal disc.