580 EMBRYOLOGY OF THE TURTLE. Part III. 
(fig. 9, e, fig. 9a, e). Subsequently, this opening lessens considerably, but is never 
closed, not even in the adult. At this period, the position of the brain in rela- 
tion to the spinal cord is remarkable: the whole of this organ, with the excep- 
tion of a small portion, the cerebellum, (PI. 12, fig. 8, ¢, 9, ¢, 9a, ¢,) next to 
the open region, trends in a direction which is perpendicular to the spinal mar- 
row (fig. 9, 9a, e). At no other time is the brain so strongly bent upon itself. 
Thus far there is but one fold, that at the cerebellum; the latter forming the 
angle of junction between the bra and the medullary tube, and all that is to 
be hereafter 

severally, the olfactory lobes, the hemispheres, the optic lobes, and 
the corpora quadrigemina—is now comprised in a simple, large chamber, which 
stands in the most deflected position conceivable, at right angles to the longitu- 
dinal axis of the body. 
Subsequently, the brain suddenly narrows behind, (PI. 12, fig. 10, ¢’,) and folds 
downward and forward along the median line, so as to give its posterior edge 
a heart-shaped figure; and thus the posterior boundaries of the corpora quadri- 
gemina are formed. We have given a perfect profile view of an embryo a little 
older, in order to show the transverse position of the brain as regards the spinal 
axis (Pl. 12, fig 6; Pl. 18a, fig. 13). Soon after this, the superior or dorsal 
side of the brain becomes indented by two transverse, shallow folds, so as to have 
an undulated profile (Pl. 18a, fig. 14). This produces a slightly three-lobed aspect; 
one lobe being anterior to the folds, one between them, and one behind the 
same; the last lobe, the corpora quadrigemina, (fig. 14, ¢’,) is clearly separated 
from the open region (fig. 14, e') behind by a folding, which we have described 
in the last phase. These folds increase in depth, and plunge far imto the head ; 
the posterior one (Pl. 24, fig. 7, ¢°) reaching much deeper than the anterior one, 
which is just m front of the eyes (fig. 7, #). The breadth of the brain, at this 
7 
stage, has decreased considerably, (Pl. 24, fig. 7a, e to e’,) and is slightly undu- 
lating at the sides, so as to appear four lobed when seen in front: the anterior 
lobe, formed by the hemispheres, (fig. 7a, ¢’,) being the same as the one seen 
before the eyes in the profile view (fig. 7); the second, the optie lobe, is that 
which lies just in front of the deepest fold (fig. 7, ¢’); the third lobe, formed by 
the corpora quadrigemina, is that which lies close behind the deepest fold; and 
the fourth and last lobe (fig. 7, ¢) is the cerebellum. The lateral constriction, 
between the optic lobes and the corpora quadrigemina, soon becomes very deep, 
(Pl. 24, fig. 9a, ¢°,) and the latter body is gradually elevated, (Pl. 14, fig. 4; 
Pl. 16, fig. 6 and 6a,) so as to give the head a crested appearance. This crest 
rises at one time very high (Pl. 14, fig. 2a; Pl. 18a, fig. 9). However, the 
prominence of the corpora quadrigemina does not indicate an absolute preponder- 
ance in size or capacity, when compared with the other lobes, since we find, upon 
