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 segments. The external appearance of the blastoderm, which remains nearly constant during segmentation, has been already well described by Leydig[79]. Footnote 79: _Rochen und Haie._ 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, 1-1/2m. in diameter. Its colour renders it very conspicuous, a feature which is further increased by its being surrounded by a narrow dark line (Pl. 6, 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 gradations into the yolk. As was mentioned in my preliminary paper (_loc. cit._), 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 examined, exhibited two furrows which 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 circumference. 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.