154 
ME. W. K. PAEKEE ON THE STEUCTUEE AND 
But the hinder part of the skull is merely a modification of the vertebral axis, whilst 
from the posterior boundary of the pituitary space the apparent continuity of structure 
has only come about by a most remarkable metamorphosis of primordial parts. The 
transversely oval space caused by the closing-in of the trabecula? and the formation of 
the “ commissure ” (Plate IV. fig. 9, joy.), has now changed into an oblong form, rounded 
in front and pointed behind (Plate V. fig. 3 , joy-)- 
We saw in the last stage (Plate IV. fig. 10, joy.) that a floor of cartilage was forming 
to the pituitary space ; this floor, derived both from the “ investing mass ” and the tra- 
becuk-e, is now complete. It no longer, however, answers merely to the floor of the 
“ sella turcica,” but is in reality the middle and anterior part of the basisplienoidal 
region, and also the whole of the presphenoidal. The outer edges of the trabeculae, 
also, are growing upwards so as to form a cartilaginous wall outside the fibrous brain- 
sac ; this, however, will be described when I come to the sectional views. 
Referring again to the side-views (Plate III. fig. 3, Plate IV. figs. 1 & 7, and Plate V. 
fig. 1), we see what morphological changes the first and second poststomals have 
undergone. The first of these, the mandibular arch, is seen to have its once free 
“ metapterygoid ” apex connected in front to the trabecula and behind to the investing 
mass. As in the last stage, this connective band grows backwards as well as outwards 
and downwards (Plate V. figs. 1, 3 & 4, and Plate VI. fig. 8, in.jog.) ; above, it is sepa- 
rated from the ear-sac by the “ foramen ovale ” (5), and the remnant of the first post- 
stomal cleft is continued as far as to the outer edge of the now dilated ear-sac. From 
this point, to below the cup for the styloid head of the hyoid cornu, the two arches, 
which were cleaving to each other in the last stage (Plate IV. fig. 7), have now entirely 
coalesced. Opposite the part where this hyoid portion of the large bar ceases, the man- 
dibular pier gives off two processes and one ray. 
The first of these processes is still, as in the last stage, a very short connective band, 
tying together the trabecula and its successor (compare Plate IV. fig. 7, jojog., with 
Plate V. figs. 1-4,^.); the pterygo-palatine bar shows as yet no signs of what it will 
become in the Frog. The other process ( or.jo .) is free, and has now grown very large, 
strongly bending upon the temporal muscle from without, and reaching up to the fore 
edge of the eye ( e ). The remaining square end of the bar (Plate V. figs. 1-5, gu.) is the 
quadrate region, with its condyle for the free ray, or Meckel’s cartilage (ink.). This 
stout, short, clubbed rod is shown in many aspects in Plate V. ; its proximal end is 
deeply and roundly notched to hinge upon the condyle of the quadrate ; it grows upwards, 
inwards, and forwards, and when the mouth is closed its upper end nearly reaches the 
trabecula; it is joined to its fellow by fibrous tissue. 
The hyoid arch has made its second great morphological change ; it has coalesced 
with the mandibular pier in front and with the auditory capsule above (Plate V. figs. 
1-4, and Plate VI. fig. 8, sJi.m., i.h.m.). The upper part, or supra-hyomandibular ( s.h.m .), 
is attached to the auditory sac much lower down and more outward than the top of the 
arch in front. 
