548 EMBRYOLOGY OF THE TURTLE. Part III. 
this will be readily explained in a little older stage, in connection with the 
development of the peripheric circulatory system. 
Soon the spinal tube (Pl. 12, fig. 11, e) loses its spatulate shape at the pos- 
terior end, by continuing to close over and growing narrower, till its extremity 
appears to vanish in a long, slender point. At the cephalic extremity, the spinal 
tube, constituting the brain, has approximated its upward folding edges, (PI. 12, 
fig. 8, &, 9, &, 9a, &,) so as to form a closed arch for a short distance along 
the region of the optic lobes, and thence to the anterior end of the head. 
Behind this closed portion, the spinal tube is yet quite open and broadly gap- 
ing, (fig. 8, 9, 9a, ¢,) as far back as a point above the heart (fig. 9, 2). 
In connection with the brain, a feature appears which has not been noticeable 
before. On each side of the ventral portion of the head, and beneath the 
region of the optic lobes, a slight protuberance (Pl 12, fig. 9 and Qa, /) stands 
out, and is rendered otherwise more conspicuous by the fact that its component 
elements are differentiated from the mass of cells about them, so as to appear like 
a clear, broad ring (fig. 9, 4) imbedded in a darker substance. From its posi- 
tion in the head, and its relation to the optic lobes of the brain, there can be 
no doubt that this is the eye. <A similar differentiation occurs on each side of 
the head, opposite the posterior part of the gap of the brain, which is still 
open. Here the clear ring, (Pl. 12, fig. 8, 9, and 9a, 24) when viewed from above, 
appears to be a cup-shaped depression, (fig. 9a, 4) from the bottom of which a 
broad band of similar substance runs toward the base of the brain. This is 
unmistakably the ear. The dorsal vertebra (Pl. 12, fig. 9 and 9a, f) are vis- 
ible, close to the ears, and extend along each side of the whole length of the 
spinal tube to its end (Pl. 12, fie. 11, /). The chorda dorsalis, now much elon- 
gated, underlies the spmal marrow, from the base of the optic lobes (Pl. 12, 
fig. Ya, g) to nearly the extreme end of the body (Pl. 12, fig. 11, g). Behind 
the vanishing part of the spinal tube (Pl. 12, fig. 11, ¢) the body is con- 
stricted considerably, and then expands into a short, oval termination, which 
contains the parts of the spinal, vertebral, and musculo-cutaneous layers that 
are not yet isolated. The sides of the body (p) are folded inwardly and down- 
wards, and the anterior and posterior edges of the ventral cavity (Pl. 12, fig. 5, 
0) are still farther approximated than before, so that, when the body is laid upon 
its back, it resembles a canoe partly decked over, with an elevated prow. 
On each side of the head, opposite the open part of the brain, three or four 
transverse furrows are visible (Pl. 12, fig. 8, m, 9, m). These extend from near 
the lower median line of the head upward, almost to a level with the edges of 
the still unclosed spimal tube, (fig. 8 and 9, ¢,) and appear to be superficial 
incisions in the musculo-cutaneous layer. These are the branchial fissures. The 
