The histological structure of the intervertebral cartilage is very distinct from that of the cartilage of the bases of the arches, the nuclei being much more closely packed. In parts, indeed, the intervertebral cartilage has almost the character of fibro-cartilage. On each side of the line of division separating two vertebræ it is invested by a superficial osseous deposit.
The next oldest larva we have had was 11 centims. in length. The filamentous dorsal lobe of the caudal fin still projected far beyond the permanent caudal fin (Plate 34, fig. 16).
The vertebral column was considerably less advanced in development than that dissected by Gegenbaur, though it shews a great advance on the previous stage. Its features are illustrated by two transverse sections, one through the median plane of a vertebral region (Plate 42, fig. 78) and the other through that of an intervertebral region (Plate 42, fig. 79), and by a horizontal section (Plate 42, fig. 77).
In the last stage the notochord was only constricted vertebrally. Now, however, by the great growth of intervertebral cartilage there have appeared (Plate 42, fig. 77) very well-marked intervertebral constrictions, by the completion of which the vertebræ of Lepidosteus acquire their unique character amongst Fishes.
These constrictions still, however, coexist with the earlier, though at this stage relatively less conspicuous, vertebral constrictions.
The gelatinous body of the notochord retains its earlier condition. The sheath has, however, undergone some changes. In the vertebral regions there is present in any section of the sheath—(1) externally, the membrana elastica externa (m.el.); then (2) the external layer of the sheath (sh.), which is, however, less thick than before, and exhibits a very faint form of radial striation; and (3) internally, a fairly thick and concentrically striated layer. The whole thickness is, on an average, 0.18 millim.
In the intervertebral regions the membrana elastica externa is still present in most parts, but has become absorbed at the posterior border of each vertebra, as shewn in longitudinal section in Plate 42, fig. 77. It is considerably puckered transversely. The sheath of the notochord within the membrana elastica externa is formed of a concentrically striated layer, continuous with the innermost layer of the sheath in the vertebral regions. It is puckered longitudinally. Thus, curiously enough, the membrana elastica externa and the sheath of the notochord in the intervertebral regions are folded in different directions, the folds of the one being only visible in transverse sections (Plate 42, fig. 79), and those of the other in longitudinal sections (Plate 42, fig. 77).
The osseous and cartilaginous structures investing the notochord may conveniently be dealt with in the same order as before, viz.: the neural arches, the hæmal arches, and the intervertebral cartilages.
The cartilaginous portions of the neural arches are still unossified, and form (Plate 42, fig. 78, n.a.) small wedge-shaped masses resting on the sheath of the notochord. They are invested by a thick layer of bone prolonged upwards to meet the dorsal processes (n.a´.), which are still formed of cartilage invested by bone.
It will be remembered that in the last stage there was no key-stone closing in the neural arch above. This deficiency is now however supplied, and consists of (1) two bars of cartilage repeated for each vertebra, but intervertebrally placed, which are directly differentiated from the ligamentum longitudinale superius, into which they merge above; and (2) two osseous plates placed on the outer sides of these cartilages, which are continuous with the lateral osseous bars of the neural arch. The former of these elements gives rise to the cartilaginous elements above the osseous bridge of the neural arch in the adult. The two osseous plates supporting these cartilages clearly form what we have called in our description of the adult the osseous roof of the spinal canal.