The neural and hæmal arches, resting on the membrana elastica externa, do not at this early stage in the least constrict the notochord. They grow gradually more definite, till the larva is five or six weeks old and about 26 millims. in length, but otherwise for a long time undergo no important changes. During the same period, however, the true sheath of the notochord greatly increases in thickness, and the membrana elastica externa becomes more definite. So far it would be impossible to distinguish the development of the vertebral column of Lepidosteus from that of a Teleostean Fish.
Of the stages immediately following we have unfortunately had no examples, but we have been fortunate enough to obtain some young specimens of Lepidosteus[521], which have enabled us to work out with tolerable completeness the remainder of the developmental history of the vertebral column. In the next oldest larva, of about 5.5 centims., the changes which have taken place are already sufficient to differentiate the vertebral column of Lepidosteus from that of a Teleostean, and to shew how certain of the characteristic features of the adult take their origin.
In the notochord the most important and striking change consists in the appearance of a series of very well marked vertebral constrictions opposite the insertions of the neural and hæmal arches. The first constrictions of the notochord are thus, as in other Fishes, vertebral; and although, owing to the growth of the intervertebral cartilage, the vertebral constrictions are subsequently replaced by intervertebral constrictions, yet at the same time the primitive occurrence of vertebral constrictions demonstrates that the vertebral column of Lepidosteus is a modification of a type of vertebral column with biconcave vertebræ.
The structure of the gelatinous body of the notochord has undergone no important change. The sheath, however, exhibits certain features which deserve careful description. In the first place the attention of the observer is at once struck by the fact that, in the vertebral regions, the sheath is much thicker (.014 millims.) than in the intervertebral (.005 millims.), and a careful examination of the sheath in longitudinal sections shews that the thickening is due to the special differentiation of a superficial part (Plate 41, fig. 69, sh.) of the sheath in each vertebral region. This part is somewhat granular as compared to the remainder, especially in longitudinal sections. It forms a cylinder (the wall of which is about .01 millim. thick) in each vertebral region, immediately within the membrana elastica externa. Between it and the gelatinous tissue of the notochord within there is a very thin unmodified portion of the sheath, which is continuous with the thinner intervertebral parts of the sheath. This part of the sheath is faintly, but at the same time distinctly, concentrically striated—a probable indication of concentric fibres. The inner unmodified layer of the sheath has the appearance in transverse sections through the vertebral regions of an inner membrane, and may perhaps be Kölliker's “membrana elastica interna.”
We are not aware that any similar modification of the sheath has been described in other forms.
The whole sheath is still invested by a very distinct membrana elastica externa (m.el).
The changes which have taken place in the parts which form the permanent vertebræ will be best understood from Plate 41, figs. 69-71. From the transverse section (fig. 70) it will be seen that there are still neural and hæmal arches resting upon the membrana elastica externa; but longitudinal sections (fig. 69) shew that laterally these arches join a cartilaginous tube, embracing the intervertebral regions of the notochord, and continuous from one vertebra to the next.
It will be convenient to treat separately the neural arches, the hæmal arches with their appendages, and the intervertebral cartilaginous rings.
The neural arches, except in the fact of embracing a relatively smaller part of the neural tube than in the earlier stage, do not at first sight appear to have undergone any changes. Viewed from the side, however, in dissected specimens, they are seen to be prolonged upwards so as to unite above with bars of cartilage directed obliquely backwards. An explanation of this appearance is easily found in the sections. The cartilaginous neural arches are invested by a delicate layer of homogeneous bone, developed in the perichondrium, and this bone is prolonged beyond the cartilage and joins a similar osseous investment of the dorsal bars above mentioned. The whole of these parts may, it appears to us, be certainly reckoned as parts of the neural arches, so that at this stage each neural arch consists of: (1) a pair of basal portions resting on the notochord consisting of cartilage invested by bone, (2) of a pair of dorsal cartilaginous bars invested in bone (n.a´.), and (3) of osseous bars connecting (1) and (2).
Though, in the absence of the immediately preceding stages, it is not perfectly certain that the dorsal pieces of cartilage are developed independently of the ventral, there appears to us every probability that this is so; and thus the cartilage of each neural arch is developed discontinuously, while the permanent bony neural arch, which commences as a deposit of bone partly in the perichondrium and partly in the intervening membrane, forms a continuous structure.