DEVELOPMENT OE THE SKULL IN STURGEONS. 
157 
Here the roots of the front teeth are cut through in the base of the upper lip (u.l., t.), 
and here the nasal pouch (ol.) has both roof and floor; the latter is continuous, now, 
with the trabecula (tr.) and bulges downwards, whilst the roof ( s.ob.) is the fore end 
of a large tract of superorbital cartilage, which grows independently of the floor for 
some distance backwards—a very common thing in the Ichthyopsida. 
This upper band is the lateral rudiment of the tegmen cranii, which in this type so 
soon covers in the great fontanelle and becomes so very massive; at present it is very 
similar to the palato-trabecular band (e.pa., tr.) below; here the trabeculae are wider 
apart than in the last section. 
Section 3.—In this third pre-oral section (Plate 14, fig. 9) the projecting fore part ol 
the mid-brain is seen to lie on the fore-brain (C 3 ., C 1 .). This is through the back part 
of the olfactory pouch (oi), which is becoming complicated; its palato-trabecular floor 
(e.pa., tr.) is, here, at its widest and most solid part, and the superorbital roof over the 
nasal sac is narrower, and is sharp at its outer edge; the roots of the front teeth are 
still in view (u.l., t.). 
Section 4.—This is through the fore part of the eye-balls (Plate 14, fig. 10, e.) ; there 
is, here, a very solid lateral ethmoidal wall uniting the superorbital band and the 
trabecula (s.ob., tr.); the latter is losing its palatine extension, outwards, and the two 
plates are still a good distance apart beneath the fore-brain (C 1 .). Here the antorbital 
wall is cut through ; it is membranous, at present. 
Section 5 .—In this section (Plate 14, fig. 11) the superorbital band (s.ob.) is simply a • 
moiety of the tegmen cranii, for the projecting part is gone, and each band is creeping 
towards its fellow over the front of the mid-brain (C 3 .). So also, below, the trabeculae 
(tr.) have lost their palatine projection and they are creeping towards each other under 
the fore-brain (C 1 .) ; these latter are the larger plates, and are rather indented below 
in the middle. Opposite the eye-ball, the front third of which is cut across, there is 
a small tract of cartilage cut through; it is rounded above and sharp below; this is a 
tract which runs backwards as far as to the Gasserian ganglion (see Plate 15, 
figs. 2, 3, al.s., V.); it is orbito-sphenoidal here (o.s.), and alisphenoidal behind. 
Section 6.—The lens of the eye (Plate 14, fig. 12, e.) is now reached, and the sclerotic 
is becoming cartilaginous; this section is very similar to the last, but the superorbital 
and orbito-sphenoidal bands (s.ob., o.s.) are thinner, and the trabeculae (tr.) are approxi¬ 
mating; here the fold of the upper lip (u.l.) is cut through; it is seen to be folded 
off from the fore face, so that at this point it seems to be attached by a narrow 
isthmus ; the next two figures will explain this'(see also Plate 13, fig. 11). 
Section 7.—This and the next (Plate 14, figs. 13, 14) are through the highest (or 
deepest) part of the brain, for here the bulging mid-brain (C 3 .) lies right over the fore¬ 
brain (C 1 .) in front of the infundibulum, which is cut through behind the second of 
these (Plate 14, fig. 1, inf.). Here the growing roof and walls of the orbital region 
of the skull are composed of three bands on each side (s.ob., o.s., o.s'.), for over the 
emerging optic nerve a lower tract of cartilage has appeared, which is thick on its 
