_nl._ Protoplasmic network in yolk. _zp._ Zona pellucida. _ch._ Structureless chorion. _fep._ Follicular epithelium. _x._ Structureless membrane external to this. EXPLANATION OF PLATES 8 AND 9. (X. p. 286.) COMPLETE LIST OF REFERENCE LETTERS. _a._ Arteries of yolk sac (red). _al._ Alimentary cavity. _alv._ Alimentary vesicle at the posterior end of the alimentary canal. _an._ Point where anus will appear. _auv._ Auditory vesicle. _bl._ Blastoderm. _ch._ Notochord. _es._ Embryo-swelling. _h._ Head. _ht._ Heart. _m._ Mouth. _mg._ Medullary groove. _mp._ Muscle-plate or protovertebra. _op._ Eye. _sc._ Segmentation cavity. _sos._ Somatic stalk. _ts._ Tail-swelling. _v._ Veins of yolk sac (blue). _vc._ Visceral cleft. I. _vc._ 1st visceral cleft. _x._ Portion of blastoderm outside the arterial circle in which no blood-vessels are present. _yk._ Yolk. PLATE 8. 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. Fig. 3. The yolk has now become completely enveloped by the blastoderm. The arterial ring has increased in size. The venous ring has vanished, owing to the complete enclosure of the yolk by the blastoderm. The point where it existed is still indicated (_y_) by the brush-like termination of the main venous trunk in a number of small branches. Fig. 4. Diagrammatic projection of the vascular system of the yolk sac of a somewhat older embryo.