i 
446 MR. W. K. PARKER OK THE SKELETON 
the cheek, is the rudimentary non-perforate 1st cleft.' 1 '' Behind this part the seven 
branchial pouches (br.p l .-br.p\) are laid open, and their main folds shown ; behind 
them is the heart ( h.) ; this section is just beyond the mid-line below, for it exposes 
the cavities of the pouches of the right side. Under the muscular segments we see 
the myelon (my.) in its theca, but the brain has been removed to show the auditory 
capsule (au.) and its meatus. In front of the capsule the trigeminal (Y.) passes 
out, and behind it, the vagus (X.). In the middle of the space between the auditory 
capsule and nasal sac ( ol .) the optic nerve (II.) escapes. There is only cartilage 
in the mid-line in front—the trabecular commissure or ethmoid (eth.); over it is 
the nasal sac (ol.) with its cartilaginous roof, and a pouch is growing downwards and 
backwards as the beginning of the “posterior nasal canal” (p.n.c.); the sac opens 
above as the external nasal aperture (e.n.). The notochord (nc.) bends downwards in 
front, and reaches to the pituitary region a little in front of the ear-capsules. 
This sectional plan will enable us to understand what follows. 
B.— Dissected head of larval P. fluviatilis, 6 inches long. 
The larva is now eighteen times the length of the embryos just described, and that 
which strikes the eye at once is that the trabeculae (Plate 19, figs. 4, 5) have united in 
front to form the rudimentary ethmoid, and that the cartilage is nearly all, in the 
cranium proper, of the hard kind—it has passed insensibly into this sort. 
The cranium is now a long loop or staple of cartilage ; the points of which, behind 
the ears, are soft, but the rest all hard cartilage. The fore end turns round by a 
sudden convexity (eth.), and the sides (tr.) are pushed in twice, first a little, where the 
optic nerves (II.) emerge, and then a second time, where they clamp the notochord (nc.) 
near, but not at, its narrow bluntly-pointed end. The lanceolate space thus enclosed 
is only properly pituitary just in front of the notochord, and under the greater, fore 
part, of this space a floor is formed, notched, behind, under which the posterior nasal 
canal is beginning to creep (see Plate 25, fig. 10, ol., p.n.c.). The lesser oval space, 
behind, lies lower than the front part of the proper membrano-cranium, and this hind 
part is the pituitary region. The under surface of the conjoined trabeculae (Plate 19, 
fig 4, tr.) is convex, but above (fig. 5), the bars, in their fore half, are tilted, and the 
upper edge is growing into a low crest. This crest becomes the ethmoidal and orbito- 
sphenoidal walls (see Plate 19, fig. 1 , etli., o.s.); it is not well seen except in ripe larvae 
(Plate 19, fig. 5) ; in younger specimens (Plate 26, figs. 5, 6) there is merely a tilting 
of the bars which have a concave upper edge. 
These rudiments of cranial walls are not seen in the alisphenoidal region, here the 
walls are entirely membranous before transformation. In the parachordal region 
* Professor Huxley (Proc. Roy. Soc., Yol. xxiii. (1874), p. 129) speaks of an external opening to this 
cleft, but neither Balfouk, Scott, nor I can find it. 
