Cuap. II. FOLDINGS OF THE EMBRYONIC DISC. 569 
In consequence of the elongation of the head, the brain (PI. 23, fig. 3) is 
also more straightened, especially the olfactory lobes and nerves, (e-c’,) so that 
from the Schneiderian membrane (c’) to the cerebellum (e¢) there is a long and 
pretty uniform curve. The olfactory lobe (c) is much more enlarged, and seem- 
ingly at the expense of the hemispheres (a). The hemispheres (a) are on a 
level with the corpora quadrigemina (4). The optic lobes, (/,) im following the 
elevation of the hemispheres, are raised considerably above the floor of the fourth 
ventricle (p). The floor of the fourth ventricle (p) is much thicker, and the 
whole ventricle is proportionably larger and broader, than heretofore. The vascu- 
lar covering (g’) of the medulla oblongata contains more bloodvessels. The entrance 
to the ear may be recognized externally as a large dark spot just behind and 
above the corner of the mouth (Pl. 18a, fig. 1). At the tip of the upper jaw 
there begins to appear a slight protuberance, (Pl. 18a, fig. 1,) the nature of which 
will be better understood in future stages. 
Besides the distinctly developed pulmonary artery, (PI. 24, fig. 10, 10a, 10b, 7, 
i’,) already mentioned, the heart has now a complete valvular apparatus (fig. 10b, 
vo, vl) between the auricles (4°, 4?) and the ventricle, (/4,) and a large venous 
sinus (fig. 10a, vs). The extent of the vascular area is variable, in some instances 
covering a large portion of the yolk, (Pl. 15, fig. 7,) and in others not more than 
one half (Pl. 15, fig. 8, 8a, 9) of its surface. The lungs are now many-cham- 
bered (Pl. 20, fig. 4, 4a); the partitions are traversed by bloodvessels, (4, 4’, ¢,) 
and new channels of blood are being hollowed in the younger partitions (a’). The 
allantois extends as far as the edge (Pl. 15, fig. 8, 2% fig. 8a, fig. 9) of the vas 
cular area, where it bends upon itself and passes upwards and follows the inner 
contour of the shell very closely. The characteristic parallel bloodvessels enable 
one to follow very easily the foldings of this organ (Pl. 14, fig. 13; Pl. 15, fig. 
8, 8a, 9). 
The skin of the neck has strong folds, (Pl. 15, fig. 9,) indicative of the retrac- 
tility of the head, which the embryo possesses in a marked degree. This, con- 
sequently, indicates the existence of active retractor muscles of the head and 
neck. Indeed, when the egg-shell is removed, young Chelydras of this age already 
snap fiercely at any thing that is brought near them. 
In a phase but slightly more advanced than the last, the protuberance at the 
end of the upper jaw (mentioned above, line 13) is here (Pl. 16, fig. 1; PI. 25, fig. 
10, 4) prolonged into a very prominent, sharp beak, covered by the soft and thick 
epidermis. The eyes are partially covered by movable eyelids (Pl. 25, fig. 10). The 
terminal joint of each toe (Pl. 25, fig 12, 6) is covered with a distinct and 
thick, transparent layer, (a,) resembling horn. The bones of the fingers (d@) are 
broader at the ends than at the middle. The bloodvessels of the feet are well 
72 
