282 PROF. W. K. PAEKEE Olf THE STEUCTUEE AND 



over the mucous membrane obliquely, outwards and backwards ; it reappears in a 

 curved hole behind the quadrate hinge (j'.f) ; these passages are the median and lateral 

 Eustachian tubes and the tympanic cavities (m.eu, l.eu, cl^). 



In the sectional views (PI. LXV. figs. 6, 7) the air-cavity in the hind part of the 

 basisphenoid {b.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 (PI. 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 {h.o), 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 (PI. LXV. figs. 7, 8, PI. LXVI. fig. 3, e.o) are now climbing up 

 the sides of the occipital arch ; they are a good distance yet from the basal piece {h.o), 

 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 {b.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 (PI. LXV. figs. 7, 8, between ii 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 (PI. LXV. figs. 7, 8), and the nasal capsules 

 have been worked out both by dissections and sections. The auditory capsules 

 (PI. 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 {aq.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, j^.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 infernus, with one upper and two 

 lower passages ; the foremost of these latter is for the facial nerve (vii), the others for 

 the auditory (viii). Mesiad of these there is the swelling caused by the cochlea, best 

 seen in the outer and lower views (PI. LXV. fig. 8, and PI. LXVI. fig. 3, ckl) The 

 large pre- and ^osfauditory nerves (v, ix-, x) pass through deep foramina, fore and aft ; 



