DEVELOPMENT OF THE SKULL IN THE PIO. 
313 
the lower edge of the frontal has sent inwards from its eave a plate which reaches the 
orbito-sphenoid— the orbital plate. The agreements and the differences seen by com- 
paring the Ornithic and Mammalian skulls are made very evident if this palatal view 
(Plate XXXIY. fig. 2) be put side by side with the figure of this region in the Ostrich’s 
embryo (“Ostrich’s Skull,” Phil. Trans. 1866, Plate vn. fig. 4) ; at this stage the con- 
formity is more remarkable than the difference. 
The dental ( dpx .) part of the pig’s premaxillary is broad and filled with tooth-sacs, 
which deeply groove it; the palatal processes (pp.x.) are slender. The approximating 
maxillaries («.) do not hide the vomer (v.) ; they are grooved by vessels down the 
middle of their palatine plate, whilst their dentary portion is hollow and shell-like, 
containing as it does large growing tooth -germs. The palatines {pa.) are ornithic , 
scarcely showing so much of the “ hard palate” as a Green Turtle {Chelone mydas). 
The pterygoids {pg-) are thin in their ascending part, and are clubbed hooks below ; 
they and the palatines both articulate with the great conjugational “ basipterygoid,” 
which here, as in the Ophidia, mainly arises from the alisphenoid ; it is, however, 
formed of true cartilage, as in all the Sauropsida in which it occurs. This part, 
the “ external pterygoid plate ” ( epg .), is a pronotochordal secondary structure ; it 
arises at its root from the side of the apex of the trabecula. These apices of this first 
pair of bars do not project outwards and backwards in the Pig as in the Kitten, nor does 
the “ basitemporal ” appear here in rudiment as the “ lingula sphenoidalis both these, 
the process and the bone, are exquisitely and most instructively displayed in the Guinea- 
pig ( Cavia aperea). The ring on which the tympanic diaphragm is stretched (ty.) is at 
present U-shaped, with its crura pointing backwards, and the larger on the outer and 
upper side ; this crus has a flat flange which looks upwards. The vomer has the same 
relative size as in the embryos of the Ostrich and the Whale (“ Ostrich’s Skull,” Plate vii. 
fig. 4, v, and Eschkicht “ On the Cetacea,” plate ii. fig. 2, V.). 
In the endoskeletal parts we have to deal with two tissues at once, hyaline cartilage 
and bone, principally endosteal at present, although rapidly gaining the surface and 
beginning to affect the perichondrium ; I shall describe it first in the dissections and then 
in the sections. In the side view (Plate XXXIY. fig. 1) the tracts that are hardening in 
the arch of the occiput are shown ; and of these there are five, namely the superoccipital 
and two pairs of exoccipitals (see also fig. 3). Moreover the superoccipital is double, 
as may be seen in a younger specimen (fig. 4, s.o.), but the two patches run into each 
other in a day or so. The ossification of the exoccipital is remarkable ; for within the 
substance of the massive condyle an epiphysial centre appears, quite distinct at first 
from the large rambling growth above (figs. 1 & 3, e.o .) ; these two points soon coalesce. 
The basioccipital ( b.o .) is best studied in a sectional view (fig. 5), but its form is seen 
from above and below (figs. 6 & 2) it is spearhead-shaped in outline and thick as to 
substance ; it is fast obliterating the notochord. The newer cartilage which underfloors 
the pituitary body is rapidly ossifying as basisphenoid (fig. 5, b.s.) ; the form of this 
centre is seen from below in fig. 2 ; this is the only bone at present in the posterior 
MDCCCLXX1V. 2 T 
