DEVELOPIMENT OF THE SkULL IN THE PIG. 



313 



'the lower edge of the frontal has sent inwards I'rom its cave a plate which readies the 

 crbito-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 XXXIV. 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 vir, fig. 4) ; at this stage the con- 

 formity is more remarkable than the difference. 



The dental [d.jpx.) 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 [mx.) 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 {p)a.) are ornifJiic, 

 scarcely showing so much of the "hard palate" as a Green Turtle [Chelone my das). 

 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 " (e.pg.), 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 Eschricht " 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 XXXIV. 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 {h.o.) is best studied in a sectional view (fig. 5), but its form i« 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, h.s.) ; the form of this 

 centre is seen from below in fig. 2 : this is the only bone at present in the posterior 



MDCCCLXXIV. 2 T 



