150 
MR. W. K. PARKER OK THE STRUCTURE AND 
notochord has become suddenly twice as wide as in the last, and it has now gained a 
lower position; the investing mass (iv.) is horn-shaped in section, and the base of each 
“ horn ” sits on the supero-lateral face of the huge notochord. The back wall of the 
auditory capsule (cm.) is cartilaginous, and at this part is some distance from the basal 
plate (iv.) ; in that interspace the ganglion of the vagus (X.) is shown. 
Here, happily, another visceral arch, the second branchial ( p.br ., e.br 2 ., c.br 2 ., b.br.) 
is exposed in one slice ; the segments are only indicated by the crowding of cells 
at the line where segmentation will take place. Within and behind the second 
branchial arch the third (c.&r 3 .) is partly seen, and the large, simple gills (g.) are 
shown fringing the folds of the section, below; they are clavate, because of the loop of 
the simple capillary vessel within. 
Section 8.—Behind the ear-capsules the notochord has a diameter only two-fifths 
less than that of the hind-brain (Plate 13, fig. 5, nc., C 3 .) ; the investing mass (iv.) 
embraces it more below, and is rising, right* and left, so as to form the lower half of the 
occipital arch (e.o.). Behind the gills, under the narrowed pharynx, the yolk-mass is 
now seen. 
A vertical section of the head in one of the larger larvae of this stage (9-| millims. 
long ; Plate 14, fig. 1) shows the low position of all the parts of the fore-brain 
(C 1 ., C lft ., inf.), the forward position of the mid-brain (C 2 .) and the huge size of 
the hind-brain (O 3 .). The cavity of the fold of the mid-brain runs equally forwards 
and upwards, and the infundibulum (inf.) looks backwards and is quite distinct from 
the pituitary rudiment (py). The notochord (nc.) nearly reaches the latter, it is 
very large, arcuate, and bent downwards in front, showing no disposition to enter 
the fold of the mid-brain in the “ post-clinoicl ” region. Two of the barbels (bb.) are 
seen to the right of the mid-line, and behind these we see the thick upper lip (u.l.) 
bounding the mouth (m.), which opens directly below the pituitary rudiment. It is a 
rather narrow passage, and has in its hinder boundary the thick lower lip (l.l.); in this 
the mandible (mk) is seen at its ventral end, and the fore end of the pterygo-quadrate 
cartilage (pg.g) is seen in the upper lip ; these cartilages lie a little to the right of 
the section, which is exactly in the middle. In the pharynx, above and behind the 
oral chink, the first three clefts (c£ 1-3 .) come into view, below these part of the yolk 
(y.), and in front of the yolk the heart (h.). 
Third Stage.—Larvae of Acipenser ruthenus from 13^ millims. to 14J millims. 
in total length. 
Amongst the more advanced larvae of this Pish only one reached the length of 
14|- millims., or of an inch ; the next in size were 1 millim. shorter. One of these 
latter was made into horizontal sections, and others were dissected; the unique 
specimen was made into vertically-transverse sections. 
The external appearance of these, next in size to the last, viz., 13^ millims., is 
extremely Selachian (Plate 12, fig. 9, and Plate 13, figs. 8, 9). The tail is more than 
