318 
MR. W. K. PARKER ON THE STRUCTURE AND 
are the sphenoidal structures complex. As in other Rodents, there is a large presphe- 
noid as well as a basisphenoid ; the alisphenoids are ossified from two centres on each 
side; long “lingular” pedicles are formed by the apices of the trabeculae, and to these 
are articulated a pair of long, outstretched falcate bones, the evident counterparts of the 
“ basitemporals ” of the Bird. In this animal also the “ external pterygoid processes ” 
axe basijnterygoids, and the small pterygoid bone is attached to their apex. Even in the 
Ruminants these spurs are basal in their origin. (For the development of the human 
sphenoid, see Huxley, ‘ Elem. Comp. Anat.’ p. 144.) 
Sixth Stage. — Embryo Pigs , 6 inches long , measured from snout to ischium. 
The head in this stage equals in size that of a Squirrel, but its bones are much more 
dense. The roof-bones (Plate XXXV. figs. 1-3) are now applied to each other edge 
to edge by sutures, and in certain places overlapping as squamae. The “ anterior fonta- 
nelle ” (fo.) is still open, but is much lessened ; the parietal and occipital bones now form 
a good “ lambdoidal suture.” The nasals, frontals, parietals, squamosals, lacrymals, pre- 
maxillaries, maxillaries, and malars (n.,f, p., sg., 1., px„, mx., j.) are all so far formed as to 
require but little change of size to fit them for their adult relationships. The palatal 
region (Plate XXXV. fig. 2) shows a great development of the secondary floor or 
“ hard palate,” the palatines themselves being now tied together at the mid line below. 
The ossification of the cartilaginous skull has advanced greatly ; the superoccipital is a 
large, strong shell of bone, and it is at present separated by a larger tract of cartilage 
from the exoccipitals than in the last stage ( s.o ., e.o.). The latter have now only one 
centre, for the epiphysis formed primarily in the substance of the condyle has coalesced 
with the outer deposit ; this is now creeping far down into the substance of the long- 
twisted paroccipital process (p.oc.). The basioccipital ( b.o .) now reaches from the 
foramen magnum to the spheno-occipital synchondrosis ; and in front of it (fig. 2) 
the basisphenoid is now a thick mass of bone (see also fig. 3, b.o., b.s.). The presphe- 
noidal region is hidden below by the vomer (fig. 2, v.), but in the section (fig. 3, p.s.) it 
is seen to be largely unossified. The alisphenoids (figs. 3 & 4, al.s.) are solidly anchy- 
losed to the median piece; they are larger relatively to the orbito-sphenoids, but 
are still inferior in size and in place; they largely owe their size, laterally, to the 
external pterygoid processes ( e.jpg .), for their cranial region is small. The whole orbital 
wing is much more contracted, relatively (figs. 1, 3, 4, o.s .) ; it has become detached 
from the ethmoid, and is some distance from the auditory mass. The two centres are 
completely anchylosed at the mid line, and quite enring the optic passages (2) ; below 
(fig. 3), they are forming the presphenoid. 
The remainder of the facial axis and nasal septum is one sheet of solid cartilage; 
and so also is the complex nasal labyrinth, now much more complex in its turbinal 
growths and cribriform plate ( u.tb ., cr.gp.). 
From the intimate impaction of the auditory mass into the sides of the cranium, its 
osseous centres have caused much confusion ; this has, however, been greatest in the 
