MR. W. K. PARKER OR THE STRUCTURE AND 
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The azygous tubular tract of skeletal tissue surrounding the notochord—membranous 
or cartilaginous, as the case may be—is stopped in front by the coalescence of the 
basal plates : they, then diverging, leave a large pituitary space—for some days 
membranous. In two places an “ inter-trabecular ” tract of cartilage appears ; behind, 
filling in this large inter-orbital pituitary space, flat and thin; and in front, over the 
coalesced parts, as a rising wall between the nasal sacs: this is the first rudiment of 
the perpendicular ethmoid and septum nasi. 
Afterwards, but not in larval life, a third inter-trabecular tract fills in the re-entering 
angle between the cornua trabeculae. 
The upgrowing wall turns over above, and forms more or less roof; this is deficient 
along the middle of the interorbital part of the skull, also often, in one, or mostly 
two, places, between the exit of the 5th nerves—these are the main and secondary 
“ fontanelles.” 
The ear-sacs get, now, a secondary floor from the basal plate; parachordal tracts 
reach to the cleft now forming in their fundus, which is becoming the fenestra ovalis, 
right and left, the membrane closing which chondrifies as the stapes. 
The nasal sacs now acquire a distinct roof of cartilage, which soon coalesces with 
the middle wall or perpendicular plate ; the ethmoidal end of the skull, also, besides 
closing in the skull-cavity, grows out as wings (“ aliethmoids ”), behind and round the 
nasal sacs. 
The quadrate end of the suspensorium sends in a spike towards the cornu trabeculae 
in front of the inner nasal opening; this is generally attached by a ligament, but some¬ 
times touches the cornu ; this is the rudiment of the pre-palatine. A ridge appears 
on the pterygo-palatine conjugation, which often grows into the orbital fenestra as a 
flap ; this is the post-palatine rudiment. 
One or two pairs of cartilaginous plates are now well-formed on the upper lip and 
answer to the anterior dorsal plate and angulo-labials of the Lamprey: and the thick 
divided, semiluminar mass of cartilage, between the free mandibles, forms the sucking 
disk (or inferior labials). 
The four subcutaneous cartilages margining the three branchial clefts have grown, 
the first and fourth into thin, baggy pouches, and the second and third into broad 
bars. These are covered with a free growth of branchial tufts, that break out between 
the clefts, but are hidden under the great membranous operculum—closed entirely on 
the right side. 
The hyo-branchial cleft is still open, but the mandibulo-hyoid never opens outside; 
over it the opercular membrane becomes cartilaginous, the cartilage growing down¬ 
wards and forwards from the upper edge of the auditory sac, or from the elbow of the 
suspensorium ; it becomes an independent spiracular cartilage. 
The massive cerato-hyals are conjugated by simple cartilage—the basi-hyal; behind 
it a pear-shaped plate appears, its base foremost; it represents two basi-branchials; 
embracing this, and running backwards there is a pair of broad, flat, hypo-branchials, 
