130 
MR, W. Iv. PARKER OK THE STRUCTURE AND 
become the basioccipital articular ring, and those above the zygapophyses for the first 
cervical vertebra. Sectional views (Plate III. fig. 11, behind , and Plate V. fig. 5, in 
front ) show the increasing thickness of the basilar plate from before backward. The 
lateral and superior parts of the occipital cincture are everywhere continuous with the 
related parts of the periotic capsules (Plate IV. figs. 1 & 2) : these structures are pecu- 
liarly* elegant at this stage, as the cartilage only thinly veils the curves and swellings of 
the membranous labyrinth, which, like the brain (fig. 4), is now very large relatively. 
The apertures for the hinder division of the fifth nerve (the front division escapes over 
the notch), the “ portio dura” of the compound seventh, and the “ vagus” and its com- 
panion, these can be seen piercing the periotic walls (figs. 1-3, 5 b , 7 a , 8). Above (figs. 
1 & 2) a gentle sulcus separates the growing roof from the elevation caused by the 
anterior and posterior canals; the latter, swelling out behind, form the “ epiotic” emi- 
nences. The most projecting part laterally is formed by the horizontal canal, and under it 
(fig. 3) is the ridge, which sets bounds externally to the facet for the extended head of 
the hyo-mandibular (figs. 1 & 3, tun.). In the middle of the under surface the skull- 
base is swollen into an oval eminence on each side, and the cartilaginous part of 
this swelling is deficient outwardly ; here still lingers the primordial “ fenestra ovalis 
and mesiad of this the cartilage is very thin (Plate V. fig. 5), for here is the “sacculus” 
with its “ otoliths.” In front the auditory sac grows into a laminar form, and this thin 
edge gradually narrows as it passes above into the “supraorbital band;” this ingrowing 
lamina becoming much more developed forwards, gives rise to the “ alisphenoid,” which 
is distinct neither from the band above nor from the ear-sac behind; it is some 
distance from the investing mass. There is at present no rudiment of even the very 
rudimentary “ basisphenoid” of the adult. The whole of the “anterior sphenoidal 
region” is membranous at this stage (Plate IV. fig. 7, o.s.). A description of the sectional 
views, although involving some recapitulation, will serve to make the description clearer. 
A longitudinal section (Plate IV. fig. 4) shows that the roof-cartilage growing back from 
the ethmoid only lies over the fore part of the “mesencephalon” (C 2 ); anteriorly the 
“ mesoethmoid” ends abruptly over the nasal sacs; over this part the thin “ supereth- 
moidal” lamina of bone ( eth .) is seen. The nasal sac ( ol .) lies upon the “ trabecular cornu,” 
and to it the olfactory crus (1) is passing; it pierces the antorbital wall. In front of the 
trabecular horn the “premaxillary” (p.x.) is cut through. The trabecular floor (tr.) is 
continued to beneath the “ thalamencephaion” (C 1") and then diverges: the pituitary 
body (py.) is seen in front of the notochord ( nc .). Beneath the trabecula is seen the 
thin parasphenoid (pa.s.), but not the “ vomer ;” this antecedence of the former bone 
brings this stage to a level in this respect with Lejndosiren. Part of the “cerebellum” 
(C 3) is covered with cartilage, the “ superoccipital” (s.o.) ; the medulla oblongata ( m.ob .) 
lies on the notochord. But the “ prosencephalon” does not lie upon the trabecular 
floor; it rests upon the wings of an “interorbital septum” ( i.o.s .), which is at present 
entirely membranous. A more enlarged view of the middle of this section (Plate IV. 
fig. 5) below shows the orbital muscles passing beneath the notochord on each side of 
