DEVELOPMENT OE THE SKULL OF THE COMMON EEOC. 
147 
does not touch the auditory sac by a distinct space ; and the third, like the four bran- 
chials, roots inward beneath the sac ; it will afterwards coalesce with it and be carried 
outward to be again segmented off in a later stage. Of the many horizontal sections 
made in my study of this stage I have figured five (Plate IV. figs. 2-6) : they dip more 
or less ; and indeed the head is as yet bent upon itself, so as to require the section to 
incline forwards. In fig. 2 the three pairs of sense-capsules have been cut through (ol., e., 
an,.), and the first cerebral vesicle, now divided into prosencephalon and deutencephalon 
(C I s , C l c ), has been so laid open as to expose part of the pituitary body (py.) ; the 
upper part of the notochord ( n.c .) is also laid bare. All that can be seen of the facial 
arches here, is the free incurved tops of the first and second — the third or hyoid lying 
in a lower plane : this view is of a solid preparation, seen from above. In fig. 3 the 
section was arched; the razor passing through the upper part of the yelk-mass ( y .) below 
the notochord, and then severing the pituitary body from the “ deutencephalon,” passed 
through the latter and the prosencephalon (C l 4 , C P ) ; this object was seen from below. 
The nasal sacs are left with the lower half of the head ; but the eyeballs (e.) are 
exactly halved ; here we have the foremost facial arches severed lower down than 
in the last figure. The next section (fig. 4) is exactly horizontal in relation to the axis 
of the head; it is made through the notochord (n.c.) with its investing mass (i.v.), the 
pituitary body (py.), the base of the “deutencephalon” (C l c ), and of the “prosence- 
phalon” (C I s ), between the eyes and nasal sacs, neither of which are displayed. The 
ear-sacs (an.) hide the tops of the third arch ; but the second and first are shown in this 
section, which is seen from above. 
In another section (Plate IV. fig. 5), which dipped considerably forwards and cleared 
the floor of the cranium, the whole circle of facial arches is shown with part of the 
notochord (n.c.), not its actual termination, however, but obliquely, near the first ver- 
tebra ; this is shown as a thin slice, and demonstrates the perfect independence of the 
bars, besides showing the completion of the branchial clefts. Compared with the first 
stage (Plate III. fig. 6), we see a great change in the trabeculse or first pair; they are 
growing towards each other in front, each showing a broad, squared end ; posteriorly the 
“ commissure” will be formed ; anteriorly the free rounded angle of each bar will grow 
into a “ trabecular horn.” The groove on the inside, between the first and second bar, 
is less deep than the next ; behind the third (hyoid) the clefts are perfect. The third 
arch (second postoral) shows its opercular fold (op.) ; and the two next have attached 
to them the roots of the palmate free gills; the branchial arches (hr. 1-4) are seen to be 
compressed in sectional form. This second stage is illustrated by another still lower 
section; this is a solid object seen from below (Plate III. fig. 13). Here the upper lip 
is cut through, and the azygous rudiment of the two upper labials (u.l.) is displayed. 
The mandibular and hyoid arches (first and second postorals) are cut through, and a 
bird’s-eye view is given of the fore part of the palate. Here the mucous membrane is 
seen folded round the first true cleft, that between the preoral and first postoral 
arches : it is not complete at present on the outer side ; but being the rudiment of the 
