592 ' EMBRYOLOGY OF THE TURTLE. Part III. 
The Skull. It is not till much later than the appearance of the vertebra 
that the cranial portion of the vertebral column makes its appearance. The ear- 
liest period at which we have noticed it was after it had risen from below, so 
as to envelope the whole brain in front of the cerebellum, (Pl. 24, fig. 7, &,) 
and had gained considerable thickness, almost twice as great as the skin just 
exterior to it. It rises gradually at the posterior part, and grows proportionally 
thinner, except at its base, (PI. 23, fig. 3, 1) where it becomes very thick. At 
this pomt it is merely cartilaginous, and extends in a uniform layer from the 
nose (fig. 3, 4) to the posterior opening at its base, (fig. 5, J,) including the 
elements of the vomer, (5,) which underlie the Schneiderian membrane (c’) 
and the space between the eyes, the ethmoid, (3,) the sphenoid, (6,) the 
basal occipital, (2,) the occipital crest, (J,) and several other bones, not indi- 
cated in the figures. The supermaxillary or upper jaw bone (fig. 3, 8) forms 
a separate cartilaginous layer. At the time the Turtle is hatched, the different 
bones of the head are generally marked out, but in very different degrees of 
ossification. The os incisivum, (Pl. 23, fig. 4, and w-e. 38, 4) or intermaxillary 
bone, although as yet very spongy, is the most ossified, and is probably the one 
most needed of any for the purposes of feeding. The upper maxillary, (fig. 11, 
and w-c. 11, d’,) the ethmoid, (fig, 4, fig 11, and w-e. 3, ”, wee. 11, d,) and 
the frontal bones (fig. 4, and w-e. 3, », fig. 10, and w-e. 10, f) are hardly less 
ossified than the mandibles. The parietal (fig. 8, and w-e. 8, 2) and the sphe- 
noid bones (fig. 8, and w-c. 8, 7) come next in hardness. The crest of the occi- 
put (fig. 4, fig. 5, and w-e. 3, n, n’, w-c. 5, m, m’) has only an external layer 
(w-c. 3, aw, w-c. 5, m’) ossified, the rest is cartilaginous; the basilar portion (fig. 4, 
and w-e. 3, 7) of this bone begins to be hardened internally. The same may be 
said of the sphenoid bone (fig. 4, fig. 8, w-e. 3, 7’, w-c. 8, 7). The other bones 
of the head are, as a general thing, merely cartilagmous. The basilar part (fig. 
4, and w-e. 5, 7) of the occipital bone does not trend in the same line with the 
sphenoid, (7’,) but is elevated to an angle of at least thirty degrees above it. 
In the adult, these two bones trend in the same line. 
The Shield’ Very few investigations have been made respecting the develop- 
ment of the different elements which compose the shield2 The manner in which 
the roof-like dorsal shield originates has already been described, (p. 562, Pl. 15, 
fig. 15,) and the changes in form through which it passes have been pointed out. 
We are not prepared to say any thing more in respect to its internal metamor- 
phoses, until the time when the young Turtle is hatched. At that time the shield 
1 Comp. Part II., Chap. 1, Sect. 4, p. 255, and 2 For more details, see Rathke, Entwickelune 
I } I g 
Sect. 5, p. 263-265. der Schildkréten, p. 84, 101, 105, and 177. 


