Cuap. II. FORMATION OF THE ORGANS. 585 
is very evident that the optic nerves (fig. 2, %, fig. 2b, %) arise directly from 
the optic thalami, (/,) and not from the corpora quadrigemina, (4,) as would 
appear from the figures of the adult brain given by Bojanus.’ 
The corpora quadrigemina (fig. 2, 4, fig. 2a, 6, fig. 2b, 4) gradually lose their 
large, open cavities, in consequence of the elevation of the lower floor, the 
aqueductus Sylvi, (fig. 2b, 9,) of this part of the brain. The volume of the 
corpora quadrigemina, (fig. 2, 0, fig. 2a, 0, fig. 2b, 4,) compared with that of the 
hemispheres, (fig. 2, a, 2a, a, 2b, a,) is at this time about as two to three, which 
is a very large proportion, compared with what it is in the adult (Pl 25; compare 
fig. 13, cg, fig. 13a, cg, with 4). The corpora quadrigemina are heart-shaped, 
with the broader end next to the hemispheres, which have a similar shape, but 
in a reverse position from that of the former. 
The cerebellum (fig. 2, e, fig. 2a, e, fig. 2b, e) continues to bulge out behind 
the corpora quadrigemina, and to gain in bulk, but at a slow rate when compared 
with the other parts of the brain. The Schneiderian membrane enlarges more 
rapidly in a vertical direction (Pl. 23, fig. 5, ¢) than horizontally, and becomes 
gradually compressed, at the sides, so as to contain a very high but narrow 
chamber. The exterior opening of the cavity of this membrane first appears as 
a narrow channel (fig. 3, 9) with a very thin wall. The pineal gland (fig. 3, d) 
becomes constricted at its base, and thus the first step is taken to form its 
pedicel. The corpora quadrigemina (fig. 3, 4) become more constricted and 
depressed at the base behind, and in consequence more shut off from the cere- 
bellum (fig. 3, e). The latter, in continuing to increase in size, gradually bends 
obliquely upward, so as to cover by degrees a considerable portion of the fourth 
ventricle (fig. 3, p). The fourth ventricle, (fig. 5, p,) by bending forward upon itself, 
allows the cerebellum (e) and the posterior end of the corpora quadrigemina (4) 
to sink into the angle formed ‘by its approximating anterior and posterior borders. 
The edges of the opening (fig. 3, ¢, f) of the medulla oblongata grow thicker, 
until in time they equal in this respect its lower wall. 
At the time the embryo is hatched, (Pl. 23, fig. 4, 4a, 5, 6, 7, 8, 9, 10) the 
brain is far from having that long, slender, and flat shape which obtains in the 
adult (Pl. 25, fig. 13, fig. 13a). In addition to what has already been pointed out, 
(p. 575,) it may be added that the Schneiderian membrane is very thick, especially 
the wall (Pl. 23, fig. 4, fig. 11, w-e. 5, 6, w-e. 11, a) facing the median line of 
the upper jaw. The channel (fig. 4, and w-e. 3, a) leading from the Schneide- 
rian membrane gradually narrows, till, at the nostrils, it opens externally with a 
1 L. H. Bojanus, Anatome Testudinis Europex, 88, ii., ii4, ii.4, with our Pl. 25, fig. 15a, and the ex- 
ete., Vilne, 1819-1821. Compare his tab. 21, fig. planation of his figure, p. 91, with ours. 
