564 EMBRYOLOGY OF THE TURTLE. Part III. 
phase of the embryo of Thalassochelys (PL 6, fig. 20) with the very broadly 
webbed feet of any of the Trionychide (Pl. 6, fig. 1-6) may easily show that 
the dense, tough, and stiff paddle of the Chelonioidx is not a retrograde metamor- 
phosis tending to the embryonic simplicity of younger stages, (Pl. 18a, fig. 9,) but 
an excessive development of that which forms the soft web in Trionychidx, and 
a hardening of its surface by the growth of closely set scales all over its surface. 
In the next phase, (Pl 14, fig: 1;. Pl 15, fig, 11,. 12, 12a; Pl 16) fig-7 55 
Pl. 17, fig: 4, 4a,.65 Pl. 21, fig. 22-267::30; Pl 23, fig.ol wal; Pl: 25) fig) 
the different regions of the brain (Pl. 23, fig. 1, la) are very distinctly marked 
out, with the exception of the olfactory lobe, (¢,) which as yet hardly forms a 
sensible swelling. As a whole, the brain has about the same proportions as 
obtain among bony Fishes, excepting that it is strongly bent upon itself, and the 
hemispheres (a) are on a much lower level than the corpora quadrigemina (4). 
The crystalline lens of the eye has already its characteristic cells, (Pl. 21, fig. 
30,) arranged in lines (4) and in concentric layers. The proportions of the body 
hold about the same relations to each other as in the last stage, excepting that 
the trunk is a little deeper in the region of the ventral opening. The back is 
more curved, the highest point of the arch being at the middle of the shield. 
The border of the shield is much lower, and does not run im a straight course 
from end to end, but curves downward, (Pl. 14, fig 1; Pl. 16, fig. 5,) reaching 
lowest just above the heart, and has a rather sinuous outline. The ribs are 
more apparent, (Pl. 14, fig. 1,) and terminate in a broad, wavy band at the edge 
of the shield, each wave being opposite to a rib. The breadth of the shield 
is greater, and has a broader ovate shape (Pl. 6, fig. 23). The edge of the 
abdominal aperture projects considerably, and simultaneously broadens, so as to form 
a trumpet-shaped umbilical passage (Pl. 14, fig. 1; Pl 15, fig. 12; Pl 16, fig. 5) 
for the allantois and neck of the yolk sac. The allantois (Pl. 15, fig. 11) is 
more extended in the dorsal region of the embryo, and covers more of the 
yolk sac; it also embraces the whole trunk of the embryo, above and_ below, 
(Pl. 15, fig. 12, 12a,) but as yet leaves the head free. The parallelism of the arte- 
ries and veins of the allantois, mentioned in a former page, (p. 560; Pl. 18a, fig. 9,) 
is here (Pl. 15, fig. 11, 12) particularly noticeable, even to the tips of the smaller 
vessels. A more highly magnified view (PI. 17, fig. 4, 4a) shows not only that 
this parallelism ceases among the very minutest vessels, the capillaries, but that 
the number of the latter is very large within a small space, and that they run 
in every possible direction. These two latter peculiarities are very different from 
what obtains in the superficial portion of the vascular area, (Pl. 15, fig. 11, 12, 
12a; Pl. 16, fig. 5; PL 17, fig. 6,) where the minutest of the bloodvessels run 
in a more or less parallel direction to each other, and are comparatively far less 
