DEVELOPMENT OF THE SKULL IN THE SALMON. 
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the pituitary body ( o.m ., py., nc .) ; this lobe does not yet reach the parasphenoid. A 
section like the fore part of the last (tig. 6), but to the right of the mid line, shows the 
opening of the nasal sac (ol.), the inner face of the eyeball (<?.); the “ ectoethmoidal,” 
or antorbital wall, is cut through external to the roof, and is seen articulating with the 
palato-pterygoid bar (p.py.), in front of which is the principal upper labial (u.l. a ); the 
premaxillary (p.x.) is cut through, and the upper jaw is seen from the inner side.* The 
first and second transverse sections (Plate III. figs. 7 & 8) have already been spoken of; 
in the latter the forks of the parasphenoid ( pa.s .) are cut through, and in the first it is 
severed in the vomerine region. In the third section (Plate III. fig. 9) the “ prosence- 
phalic” lobes are cut through; and this part of the membranous cranium is roofed over 
by the ethmoidal “ tentorium,” the free edges of which are bound down by the styloid 
“frontal” (f). The cranial walls pass down into the “ interorbital septum” (i.o.s.), which 
is continuous below with the perichondrium of the tilted and coalesced “trabeculae” (tr.). 
The parasphenoid here is arched and outspread laterally, in accordance with the lower 
surface of the trabecular floor. The huge eyeballs ( e .) are separated by copious gelatinous 
tissue, keeping the olfactory crura (1) from the septum, which is here at its highest. 
The fourth section (Plate IV. fig. 7) is through the “ thalamencephalon” (Cl") and the 
fore part of the “mesencephalon” (C2), on each side of which the tent-ropes (“super- 
orbital bands”) ( s.ob .) have been severed behind the rudimentary frontals ; as this section 
is through the eye behind the entrance of the optic nerve and in front of their exit from 
the cranium, they are seen in section here (2) near the lowered interorbital septa. The tra- 
becular floor is flatter here, and so also is the parasphenoid which underlies it (tr. pa.s.). 
The fifth section (Plate III. fig. 10) is very instructive also; it is through the middle of 
the “mesencephalon” (C 2), the fore part of the pituitary body ( py .), the most diverged 
and terete part of the trabeculae (tr.), the back of the orbit (or.), and cuts through that 
part of the “superorbital band” (s.o b.) which is passing insensibly into the auditory 
sac, its “ sphenotic” region; a downward growth from this part of the cartilaginous skull 
gives rise to the “ alisphenoid.” The exquisite infoldings of the parasphenoid (pa.s.) 
are here seen in relation to the pituitary body, exactly between the elbowed trabeculae 
(tr.). The sixth section (Plate Y. fig. 5) has been already partly described; it exposes 
the arch of the anterior and the ampulla of the horizontal semicircular canals, the “ utri- 
culus” and the “ sacculus ;” it shows the deficiency of the cranial roof at this place over 
the “ cerebellum,” and the huge size of the medulla oblongata (C 3, m.oib.). The seventh 
section (Plate III. fig. 11) is through the basioccipital, just missing the supraoccipital 
roof; it passes through the medulla oblongata (m.ob.), close behind the cerebellum: 
running askance, the razor has opened the ampulla of the left posterior canal, and has laid 
bare the canal through which the compound eighth nerve passes (p.s.c. 8). This section 
is from a less advanced specimen than that illustrated in figs. 2 & 3 ; the other structures 
severed in these sections will help to explain the movable machinery of the mouth and 
throat, which must now be described. In the profile and bird’s-eye views of the pri- 
mordial skull at this stage I have purposely excluded the delicate ectosteal laminae, many 
* 
