We have figured an embryo of 11 millims. in length, shortly after hatching (Plate 34, fig. 12), the most important characters of which are as follows:—The yolk-sack, which has now become much reduced, forms an appendage attached to the ventral surface of the body, and has a very elongated form as compared with its shape just before hatching. The mouth, as also noticed by Agassiz, has a very open form. It is (Plate 34, fig. 13, m.) more or less rhomboidal, and is bounded behind by the mandibular arch (mn.) and laterally by the superior maxillary processes (s.mx). In front of the mouth is placed the suctorial disc (s.d.), the central papillæ of which are arranged in groups. The opercular fold (h.op.) is very large, covering the arches behind. A well-marked groove is present between the mandibular and opercular arches, but so far as we can make out it is not a remnant of the hyomandibular cleft.
The pectoral fins (Plate 34, fig. 12, pc.f.) are very prominent longitudinal ridges, which, owing to their being placed on the surface of the yolk-sack, project in a nearly vertical direction: a feature which is also found in many Teleostean embryos with large yolk-sacks.
No traces of the pelvic fins have yet become developed.
The positions of the permanent dorsal, anal, and caudal fins, as pointed out by Agassiz, are now indicated by a deposit of pigment in the embryonic fin.
In an embryo on the sixth day after hatching, of about 15 millims. in length, of which we have also given a figure (Plate 34, fig. 14), the following fresh features deserve special notice.
In the region of the head there is a considerable elongation of the pre-oral part, forming a short snout, at the end of which is placed the suctorial disc. At the sides of the snout are placed the nasal pits, which have become somewhat elongated anteriorly.
The mouth has lost its open rhomboidal shape, and has become greatly narrowed in an antero-posterior direction, so that its opening is reduced to a slit. The mandibles and maxillary processes are nearly parallel, though both of them are very much shorter than in the adult. The operculum is now a very large flap, and has extended so far backwards as to cover the insertion of the pectoral fin. The two opercular folds nearly meet ventrally.
The yolk-sack is still more reduced in size, one important consequence of which is that the pectoral fins (pc.f.) appear to spring out more or less horizontally from the sides of the body, and at the same time their primitive line of attachment to the body becomes transformed from a longitudinal to a more or less transverse one.
The first traces of the pelvic fins are now visible as slight longitudinal projections near the hinder end of the yolk-sack (pl.f.).
The pigmentation marking the regions of the permanent fins has become more pronounced, and it is to be specially noted that the ventral part of the caudal fin (the permanent caudal) is considerably more prominent than the dorsal fin opposite to it.