STRUCTURE AND DEVELOPMENT OP LEPIDOSTEUS. 
397 
dorsal processes, may be reckoned as parts of the neural arches. Between the dorsal 
processes of the two sides is placed a median rod of cartilage, which is developed sepa¬ 
rately from the true neural arches, and which constitutes the median spinous element 
of the adult. Immediately below this rod is placed the ligamentum longitudinale 
superius. There is now a commencement of separation between the dorsal and ven¬ 
tral parts of the hrnmal arches, not only in the tail, but also in the trunk, where they 
pass ventralwards on each side of the body cavity, immediately outside the peritoneal 
membrane, along the lines of insertion of the intermuscular septa. These are obviously 
the ribs of the adult, and there is no break of continuity of structure between the 
haemal processes of the tail and the ribs. In the anterior part of the trunk the ribs 
pass outwards along the intermuscular septa till they reach the epidermis. Thus the 
ribs are originally continuous with the haemal processes. Behind the region of the 
ventral caudal fin the two haemal processes merge into one, which is not perforated 
by a canal. 
Each of the intervertebral rings of cartilage becomes eventually divided into two 
parts, and converted into the adjacent faces of contiguous vertebrae, the curved line 
where this will be effected being plainly marked out. These rings are united with the 
neural and haemal arches of the vertebrae next in front and behind. As these rings 
are formed originally by the spreading of the cartilage from the primitive neural and 
haemal processes, the intervertebral cartilages are clearly derived from the neural and 
haemal arches. The intervertebral cartilages are thicker in the middle than at their 
two ends. 
In our latest stage (11 centims.), the vertebral constrictions of the notochord are 
rendered much less conspicuous by the growth of the intervertebral cartilages giving 
rise to marked intervertebral constrictions. In the intervertebral regions the mem- 
brana elastica externa has become aborted at the posterior border of each vertebra, and 
the remaining part is considerably puckered transversely. The inner sheath of the 
notochord is puckered longitudinally in the intervertebral regions. The granular 
external layer of the sheath in the vertebral regions is less thick than in the last 
stage, and exhibits faint radial striations. 
Twt> closely approximated cartilaginous elements now form a keystone to the neural 
arch above: these are directly differentiated from the ligamentum longitudinale superius, 
into which they merge above. An osseous plate is formed on the outer side of each of 
these cartilages. These plates are continuous with the lateral osseous bars of the 
neural arches, and also give rise to the osseous roof of the spinal canal of the adult. 
Thus the greater part of the neural arches is formed of membrane bone. The 
hsemal arches are invested by a thick layer of bone, and there is also a continuous 
osseous investment round the vertebral portions of the notochord. The intervertebral 
cartilages become penetrated by branched processes of bone. 
