584 EMBRYOLOGY OF THE TURTLE. Part III. 
small aperture. The walls of the olfactory bulb (fig. 4a, and w-ce. 4, d) are 
very thick, especially at the constriction between the bulb itself and the hemi- 
spheres (e). This bulb is, moreover, larger and more distinguishable from the 
hemispheres than in the adult (Pl 25, fig. 13, 06, fig. 13a, 0b). The walls of 
the hemispheres (Pl. 23, fig. 4, fig. 4a, fig. 8, fig. 9, and w-c. 3, e, w-e. 4, ¢ w-c. 8, 
a, w-c. 9, a) are very thick, especially below and at the sides, where the corpora 
striata are forming (fig. 8, and w-c. 8, a, w-c. 9, a); so that, in the latter case, 
they equal the transverse diameter of the cavity (w-c. 8, d@) which they inclose. 
The walls of the optic thalami also (fig. 4, fig. 4a, fig. 8, fig. 9, w-e. 3, h, w-e. 
8, f, f') are very thick, fully as much so at the upper part as in the hemi 
spheres, and leave only a very narrow cavity between them. At the inferior 
commissure (fig. 8, and w-e. 8, g) the wall is quite thin. The extreme lower 
end of this lobe is quite pointed; but there is yet no indication of a hypophysis, 
such as may be seen in the adult brain (PI. 25, fig. 13a, 7). The corpora quadri- 
gemina (PI. 23, fig. 4, fig. 4a, fig. 7, w-c. 3, g, g’, w-c. 4, g, w-c. 7, a, a) have sunk 
so low that there is only a shallow space (fig. 7, and w-c. 7, 6) between them 
and the floor of the aqueductus Sylvii. The walls (fig. 4, fig. 4a, fig. 7, and 
w-c. 3, 7, w-c. 7, ¢, ¢) of the aqueeductus Sylvii are very thick, but do not equal, 
in this respect, those of the hemispheres and of the optic thalami. The view 
given in fig. 7, and w-e. 7, ¢ ¢ is that of a transverse section of the brain at 
a point, in the aqueductus Sylvii, which includes both the lateral and the ante- 
rior walls; hence their apparent, great thickness. 
The aperture in the upper side of the medulla oblongata is nearly filled by 
the oblong mass of the arachnoid plexus (fig. 4, fig. 4a, fig. 6, and w-e. 3, #, 
w-e. 4, k', wee. 6, 6, 0’). At the edge of this aperture the wall terminates rather 
abruptly, (fig. 6, and w-e. 6, ¢,) except that at the posterior end, where the oppo- 
site walls meet, it comes to a sharp edge (fig. 4, fig. 4a, and w-e. 3, /', w-e. 4, 
f). The spinal marrow, although a closed tube with a very small channel, (fig. 
5, and w-e. 5, ¢,) close up to the medulla oblongata, has yet a deep median 
dorsal sulcus, (fig. 5, and w-e. 5, 0,) at least as far back as the base of the 
skull. As regards the spinal marrow, nothing more can be said now beyond what 
has already been stated above. A few additional remarks respecting its Histology 
may be found in the next section. 
The Chorda dorsalis. Whatever further details are necessary in regard to this 
body may also be found in the section on Histology. 
The Eye. The eye commences to form by a bulging (PI. 12, fig. 9a, &) of 
the inferior and lateral sides of the optic lobe. When the head is seen from 
the side, the wall of this protuberance appears as a clear, bright, thick ring (PI. 
12, fig. 9, %). For a short time the hernia from the optic lobe continues to 


