DEVELOPMENT OF THE SKULL IN THE PIQ. 



321 



Tlic fourth section (fig. 9) instructs us how the olfactory labyrinth ends, as the 

 sphenoidal sinus, on each side of the presphenoid ; and it is seen that the fore edge of 

 the orbito-sphenoids wall in this region, and underprop the thin descending orbital plate 

 of the thick arching frontals (o.s.,f.). On each side is the eye-socket, and below the 

 presphenoid is the thin part of the vomer (v.) ; the palatines (^Ja.) are here cut through 

 behind the hard palate, and opposite them a section of the malar bone and lower jaw 

 is shown (./., d.). 



The fifth section (fig. 10) brings a number of bones into view; the presphenoidal 

 (trabecular) cartilage is rapidly ossifying from the coalesced orbito-sphenoidal centres 

 (o.5.,p.s.); beneath this is the vomer and on each side, protecting the hinder 



nostrils, we see the thin part of the jialatine and pterygoid {i^a., ])g.) ; outside these is 

 the fore-bent wing of bone known as the " external pterygoid plate " {e.pg.). 



The sixth section (fig. 11) is through the postero-external part of the external pterygoid 

 {e.pg.) ; it binds strongly against the inner face of the articular (" glenoid") part of the 

 many-spurred squamosal {s^.). Here the basisphenoid is cut through behind the 

 "wings," and the internal carotids mount up here to reach the " circle of Willis ;" here 

 the two limbs of the tympanic, the " os bullae," and the stylohyal {ty., o.h., st.h.) are 

 cut through. The "glenoid hinge " is here with its meniscus, and the articular region 

 of the lower jaw is still largely cartilaginous beyond the head of the articular surface. 



The seventh section (fig. 12) is through the lower edge of the parietal {p.), and also 

 through the upper and lower edges of the squamosal {sq.) above, where it binds upon 

 the mastoid region, and below, where it flanks the long paroccipital process (jp.oc). 

 The extremity of the sinuous, creeping tympanic cavity is here seen (ty.), and outside it 

 the stylohyal and tympanic (st.h., ty.) are severed ; below these is a section of the par- 

 occipital spur. Over the incus (?'.) the horizontal canal is seen, and below the stylohyal 

 the portio dura . (7"). Inside the upper and outer portion of the rambling prootic 

 centre the portio mollis (7*) is seen streaming in ; the inner face of the cochlea has 

 the prootic in its edge, and the outer and lower the opisthotic {pro., op.). The basi- 

 occipital {b.o.) is seen to have a subcrescentic form in section, and the cartilage at the 

 edges of this elegant "basilar plate" will have its outer edge ossified by the exoccipitals. 



Seventh Stage. — Neio-horn Pigs. 



vSince the last stage the skull has almost doubled its length, "and the process of ossi- 

 fication has gone on very rapidly ; moreover the form of the entire skull has become 

 much more specialized. 



Wishing to limit the illustrations to this paper, I have given but few figures of the; 

 preparations made for my own research ; but the head at this stage is most easily 

 obtained by those interested in these studies. Moreover the semi-adult condition of the 

 skull, prior to any extensive anchylosis, will be fully illustrated. Besides the finish given 

 to the general form of the skull by the now complete investing bones, the endoskeletal 

 parts are well ossified. The whole of the occipital arch and its paroccipital processes, 



MDCCCLXXIV. 2 u 



