282 PROF. W. K. PARKER ON THE STRUCTURE AND 
over the mucous membrane obliquely, outwards and backwards; it reappears in a 
curved hole behind the quadrate hinge (q.c); these passages are the median and lateral 
Eustachian tubes and the tympanic cavities (m.eu, l.eu, cl"). 
In the sectional views (Pl. LXV. figs. 6, 7) the air-cavity in the hind part of the 
basisphenoid (0.s) is shown, but not the opening below, as the section is more than half 
of the head; in fig. 6 the opening into the tympanic cavity (ty.c) is shown. These 
passages will be illustrated in their further development in the later stages. 
The chondrocranium (Pl. LXV. figs. 6,7, 8) is very similar to what has been described 
in the last stage; but some of the main bony tracts have appeared in the hind skull. 
The first of these is the basioccipital (6.0), which reaches more than halfway from the 
condyle to the pituitary space (py), but leaves a considerable tract of cartilage, behind, 
untouched. 
The exoccipitals also (Pl. LXV. figs. 7,8, Pl. LXVI. fig. 3, ¢.0) are now climbing up 
the sides of the occipital arch; they are a good distance yet from the basal piece (0.0), 
reach nearly to the top of the foramen magnum, and just touch the opisthotic region 
in front; the supraoccipital and the periotic regions are not ossified. 
The next bone is the basisphenoid (4.s); this occupies the bottom of the pituitary 
cup, and runs backwards a little below; it is not of greater extent than the spheno- 
occipital synchondrosis behind it, and it does not reach far into the “ postclinoid 
wall”? (p.cl). 
The description just given of the chondrocranium of the last stages might serve also 
for this, except that the passages for the lesser cranial nerves are more perfectly 
bounded by cartilage (Pl. LXV. figs. 7, 8, between m and v); in this stage I have 
studied the skull by sections. Here, also, I have figured the auditory capsules on 
the outer and inner sides, in lateral views (Pl. LXV. figs. 7, 8), and the nasal capsules 
have been worked out both by dissections and sections. The auditory capsules 
(Pl. LXV. figs. 7, 8) are quite unossified; they form relatively very large pyriform 
masses, With sinuous surfaces arising from the form of the membranous labyrinth 
within. 
On their inner face (fig. 7) where the anterior canal dilates at its junction with the 
posterior, there is a crescentic aperture whose concavity looks upwards and backwards; 
this is the remnant of the original involution (ag.v). In front of this there is a large 
arched swelling caused by the anterior canal, and behind it another of less extent 
caused by the posterior-canal; these are also seen on the outside (fig. 8, p.s.c). A 
gently sulcate tract separates the arched part on the inside from the swelling caused 
by the “sacculus ;” then comes the shallow meatus internus, with one upper and two 
lower passages; the foremost of these latter is for the facial nerve (vm), the others for 
the auditory (vim). Mesiad of these there is the swelling caused by the cochlea, best 
seen in the onter and lower views (Pl. LXV. fig. 8, and Pl. LXVI. fig. 3, chl) The 
large pre- and postauditory nerves (v, 1X, X) pass through deep foramina, fore and aft; 
