DEVELOPMENT OE THE SKULL IN STURGEONS. 
145 
Second Stage.—Larva of Acipenser ruthenus, from 8^ to 9-| millims. in length. 
The larvse at this stage have a very Selachian appearance (Plate 12, figs. 4-8); alto¬ 
gether they have made a very great advance in development since the last stage : I 
shall describe first the external form, then a dissection of the visceral arches, and after 
that a series of transverse sections. 
The azygous fins are beginning to be subdivided (fig. 7) ; now, the axis (notochord) 
turns downwards behind, but the tail is still diphycercal; it has become almost as long 
as the trunk, measuring from the anal aperture [an.), so that the post-anal region is 
very much longer, relatively, than in the last stage (fig. 2). The azygous fin is more 
definite along the trunk, almost reaching as a crest to the head, and the pectoral fins 
(pf) that then were at most mere thickenings of the tissue, are now w^ell-developed, 
free, auriform paddles. 
The yolk-mass (st.) makes only half the bulging it did : it is now quite portable; the 
intestine [in., an.) is now seen as a definite ventral cavity behind the stomach and yolk, 
and is nearly as long as the wide part in front. 
The heart (figs. 5 and 7, hi) is now fairly behind the operculum (op.), but is still seen 
from the outside as a double swelling. 
The mesocephalic flexure is less, and the various parts of the brain (C 1 ., C 3 ., C 3 .) are 
very visible through the diaphanous membrano-cranium.* So also the auditory organs 
show clearly through the skin, and the common tube of the anterior and posterior 
canals (am., p.s.c.), and the single lesser tube forming the horizontal canal (h.s.c.), are 
very apparent without dissection. 
The eye-ball (e.) is rapidly finishing and its socket is being formed, and there is a 
distinct suborbital ledge. The olfactory sac (ol.) is lateral and close to the eye, it has 
acquired a very definite superficial membranous capsule; this is sending downwards 
a triangular flap which tends to subdivide the opening. 
The “ barbels ” (bb.) are now well formed, and look in the larger specimens of this 
stage like formidable tusJcs; they arise from a recess in front of the upper lip, which 
has over it a fold of the fore face, and in front of it, below the fore brain (Plate 12, 
fig. 5), there is between the olfactory sacs (ol.) a very remarkable concavity; it is 
nearly circular, is as large as one of the olfactory sacs, is almost as deep, and has in 
front of it a crescentic fold, and on each side a lateral lip-like margin. Its somewhat 
produced hind margin lies between the two inner barbels, and its lateral walls run up 
to those barbels. In the lesser larvae of this stage (figs. 4, 5, m.) the mouth is still 
somewhat rhomboidal, but in the larger (fig. 7) its edges come very near together. 
The lips are now thick and large, the front is emarginate, the hind lip has an apiculate 
lobe, at the mid-line, looking forward ; each is armed with the small, clear, pointed 
teeth (t.). 
* Through want of proper food these larva? soon began to starve. Moreover, the spirit in which, 
they were preserved contracted their tissues; the figures purposely show this contracted appearance. 
MDCCCLXXXII. rr 
