-54 MR. W. K. PARKER OX THE STRUCTURE AND 



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 j^iiuitari/ space — for some days 

 membranous. In two places an " inter-trabecular " tract of airtilage appeal's; 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 laidiment 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 comua trabeculse. 



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, betv/een 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 trabeculse 

 in front of the inner nasal opening; tliis 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 jaterygo-palatiue conjugation, which often grows into the orbital fenestra as a 

 flap ; this is the post-palatine nxdiment. 



One or two pairs of cartilaginous plates ai"e 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 tliree 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 gi'owth 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 ruiming backwards there is a pair of broad, flat, hypo-branchials, 



