Fig. 52. Embryos of Acipenser belonging to two stages viewed from the dorsal surface. (After Salensky.)
Fb. fore-brain; Mb. mid-brain; Hb. hind-brain; cp. cephalic plate; Op. optic vesicle; Auv. auditory vesicle; Olp. olfactory pit; Ht. heart; Md. mandibular arch; Ha. hyoid arch; Br´. first branchial arch; Sd. segmental duct.
The epiblast at the sides of the brain constitutes a more or less well-defined structure, which may be spoken of as a cephalic plate ([fig. 52] A, cp). From this plate are formed the essential parts of the organs of special sense. Anteriorly the olfactory pits arise ([fig. 52] B, Olp) as invaginations of both layers of the epiblast. The lens of the eye is formed as an ingrowth of the nervous layer only, and opposite the hind-brain the auditory sack ([fig. 52] A and B, Auv) is similarly formed from the nervous layer of the epiblast. At the sides of the cephalic plate the visceral arches make their appearance; and in [fig. 52] A and B there are shewn the mandibular (Md), hyoid (Ha) and first branchial (Br´) arches, with the hyomandibular (spiracle) and hyobranchial clefts between them. They constitute peculiar concentric circles round the cephalic plate; their shape being due to the flattened form of the embryo, already alluded to.
While the above structures are being formed in the head the changes in the trunk have also been considerable. The mesoblastic plates at the junction of the head and trunk become very early segmented, the segments being formed from before backwards ([fig. 50] B). With their formation the trunk rapidly increases in length. At their outer border the segmental duct ([fig. 50] B, and [fig. 52] A, Sd) is very early established. It is formed, as in Elasmobranchs, as a solid outgrowth of the mesoblast ([fig. 51], Wg); but its anterior extremity becomes converted into a pronephros ([fig. 57], pr.n).
Before hatching, the embryo has to a small extent become folded off from the yolk both anteriorly and posteriorly; and has also become to some extent vertically compressed. As a result of these changes, the general form of its body becomes much more like that of an ordinary Teleostean embryo.
The general features of the larva after hatching are illustrated by [figs. 53], [54] and [55]. [Fig. 53] represents a larva of about 7 mm. and [fig. 54] a lateral and [fig. 55] a ventral view of the head of a larva of about 11 mm.
Fig. 53. Larva of Acipenser of 7 mm., shortly after hatching.
ol. olfactory pit; op. optic vesicle; sp. spiracle; br.c. branchial clefts; an. anus.
There are only a few points which call for special attention in the general form of the body. In the youngest larva figured the ventral part of the hyomandibular cleft is already closed: the dorsal part of the cleft is destined to form the spiracle (sp). The arch behind is the hyoid: on its posterior border is a membranous outgrowth, which will develop into the operculum. In older larvæ, a very rudimentary gill appears to be developed on the front walls of the spiracular cleft (Parker), but I have not succeeded in satisfying myself about its presence; and rows of gill papillæ appear on the hyoid and the true branchial arches ([figs. 54] and [55], g). The biserially-arranged gill papillæ of the true branchial arches are of considerable length, and are not at first covered by the operculum; but they do not form elongated thread-like external gills similar to those of the Elasmobranchii.