Fig. 15. Section through germinal disc of a Pristiurus embryo during the segmentation.
n. nucleus; nx. nucleus modified prior to division; nx´. modified nucleus in the yolk; f. furrow appearing in the yolk adjacent to the germinal disc.
At the close of segmentation the blastoderm forms a somewhat lens-shaped disc, thicker at one end than at the other; the thicker end being the embryonic end. It is divided into two strata—an upper one, the epiblast—formed of a single row of columnar cells; and a lower one, the primitive hypoblast, consisting of the remaining cells of the blastoderm, and forming a mass several strata deep. These cells will be spoken of as the lower layer cells, to distinguish them from the true hypoblast which is one of their products.
Fig. 16. Two longitudinal sections of the blastoderm of a Pristiurus embryo during stages prior to the formation of the medullary groove.
ep. epiblast; ll. lower layer cells or primitive hypoblast; m. mesoblast; hy. hypoblast; sc. segmentation cavity; es. embryo swelling; n´. nuclei of yolk; er. embryonic rim. c. lower layer cells at the non-embryonic end of the blastoderm.
A cavity very soon appears in the lower layer cells, near the non-embryonic end of the blastoderm, but the cells afterwards disappear from the floor of this cavity, which then lies between the yolk and the lower layer cells ([fig. 16] A, sc). This cavity is the segmentation cavity equivalent to that present in Amphioxus, Amphibia, etc. The chief peculiarity about it is the relatively late period at which it makes its appearance, and the fact that its roof is formed both by the epiblast and by the lower layer cells. Owing to the large size of the segmentation cavity the blastoderm forms a thin layer above the cavity and a thickened ridge round its edge.
Fig. 17. Longitudinal section through the blastoderm of a Pristiurus embryo of the same age as fig. 28 B.
ep. epiblast; er. embryonic rim; m. mesoblast; al. mesenteron.