Fig. 46. Section through germinal disc of a Pristiurus embryo during the segmentation.
n. nucleus; nx. nucleus modified prior to division; nx´. modified nucleus of the yolk; f. furrow appearing in the yolk adjacent to the germinal disc.
The result of segmentation is the formation of a lens-shaped mass of cells lying in a depression on the yolk. In this a cavity appears, the homologue of the segmentation cavity already spoken of. It lies at first in the midst of the cells of the blastoderm, but very soon its floor of cells vanishes, and it lies between the yolk and the blastoderm. ([Fig. 47] A.) Its subsequent history is given in a future Chapter.
Segmentation proceeds in Osseous Fishes in nearly the same manner as in Elasmobranchii. In some cases the germinal disc is small as compared with the yolk, in other cases it is almost as large as the yolk. The only points which deserve special notice are the following: (1) Nuclei, precisely similar to those in the Elasmobranch yolk, appear in the protoplasmic matter around the germinal disc; (2) After the deposition of the ova there is present in some forms a network of protoplasm extending from the germinal disc through the yolk[48]. At impregnation this withdraws itself from the yolk. It is to be compared to the protoplasmic network of the Elasmobranch ovum.
Fig. 47. Two longitudinal sections of the blastoderm of a Pristiurus embryo at stages prior to the formation of the medullary groove.
ep. epiblast; ll. lower layer cells; m. mesoblast; hy. hypoblast; sc. segmentation cavity; es. embryo swelling; n´. nuclei of yolk; er. embryonic rim.
There are two types of meroblastic ova. In one of these (Aves, Elasmobranchii) the germinal disc is formed in the ovarian ovum. In the second type the germinal disc is formed after impregnation by a concentration of the protoplasm at one pole. This concentration is analogous to what has already been described for Anodon and other Molluscan ova (p. [100]).