Fig. A. Surface view of blastoderm of Pristiurus hardened in chromic acid.
Fig. B. Surface view of fresh blastoderm of Pristiurus.
Figs. C, D, E, and F. Pristiurus embryos hardened in chromic acid.
Fig. G. Torpedo embryo viewed as a transparent object.
Figs. H, I. Pristiurus embryos viewed as transparent objects.
Fig. K. Pristiurus embryo hardened in chromic acid.
The remainder of the figures are representations of embryos of Scyllium canicula hardened in chromic acid. In every case, with the exception of the figures marked P and Q, two representations of the same embryo are given; one from the side and one from the under surface.
Plate 9.
Fig. 1. Yolk of a Pristiurus egg with blastoderm and embryo. About two-thirds of the yolk have been enveloped by the blastoderm. The embryo is still situated at the edge of the blastoderm, but at the end of a bay in the outline of this. The thickened edge of the blastoderm is indicated by a darker shading. Two arteries have appeared.
Fig. 2. Yolk of an older Pristiurus egg. The yolk has become all but enveloped by the blastoderm, and the embryo ceases to lie at the edge of the blastoderm, owing to the coalescence of the two sides of the bay which existed in the earlier stage. The circulation is now largely developed. It consists of an external arterial ring, and an internal venous ring, the latter having been developed in the thickened edge of the blastoderm. Outside the arterial ring no vessels are developed.