538 
ME. W. E. PAEKEE ON THE STEUCTUEE AND DEVELOPMENT 
large, placed crosswise, and the bars are so small and thin that each cell can be 
counted. 
The only arch that is subdivided is the mandibular, the pier or upper part of which 
is still granular, indifferent tissue ; so also are the trabeculae (Plate 22. fig. 1, q, tr.). 
There is no cartilage in any of the sense-capsules ; the whole cranial cavity is enclosed 
by a “ membrano-cranium,” the first morphological stage of this important chamber. 
I was not able to work out the cranial nerves ; but safe landmarks were found in solid 
masses of nerve-cells, from which were developing the Gasserian ganglion of the fifth, 
and, npt distinct from it to my view, the geniculate ganglion (5, 7); whilst behind and 
mesiad of the anditory sac ( au .) there is the large ganglion of the glossopharyngeal and 
vagus nerves. 
As this specimen was in advance of that whose outward appearance has been just 
described, the head had become almost straight again ; hence the three cerebral regions 
are now in almost one plane. 
Yet the fore brain (C 1 ) is in front of the mouth (m.) ; the mid brain (C 2 ) behind 
both mouth and eyes (e.). The floor of the hind brain has all been removed to show 
the subjacent structures. 
The most important of these is the notochord ( nc .), the dip downward of whose apex 
is not shown in the figure, nor the ascending process of its sheath (see Plate 21. 
fig. 4, nc.). 
Of this massive notochord two thirds belong to the skull and one third to the neck. 
The two first pairs of muscular segments belong also to the head, but the occipital arch 
is mere membrane. The investing mass or parachordal cartilages, right and left, w r ill 
be best seen in my fourth stage ; but this notochord is embraced by the trabeculse, and 
full half of the cranial notochord will be enveloped by cartilage which is the hinder half 
of the trabeculse. 
These small, non-cartilaginous trabeculae are attached, like the horns of an ox, to the 
fore end of the notochord ; they are rounded in front and flat behind. The space whose 
sides are enclosed by these little rods is not the mere counterpart of the proper 
“pituitary space;” it is the outline of the vesicle of the third ventricle (or thalamen- 
cephalon) which is here marked out. The infundibulum has been cut away from the 
pituitary body (py.), wkich takes up but little of the room embraced by the trabeculae. 
Hence we see that the sudden arrest at this part of the axial structures has arisen from 
something more than the suspension of the pituitary body ; the vesicular mid brain has 
been a still more important factor. 
The morphological meaning of the modified, stunted, non-segmentary axis, and espe- 
cially of the cartilaginous rods that grow forwards in front of the notochord, is still 
an unsolved mystery*. 
* The anterior part of the developed trabeculae for several years seemed to me to he visceral or pleural; the 
base of the septum nasi, the cornua trabeculae, and the prenasal cartilage, together, appeared to be the founda- 
tion of the “ intermaxillary arch.” (See “ Ostrich’s Skull,” Phil. Trans. 1866, p. 122.) 
