Fig. 315. Section through the vertebral column of Ammocœtes. (From Gegenbaur.)
Ch. notochord; cs. notochordal sheath; m. spinal cord; a. aorta; v. cardinal veins.
Between the membrana elastica externa and the sheath of the notochord a layer of cells becomes interposed ([fig. 316], n), which lie in a matrix not sharply separated from the sheath of the notochord. The cells which form this layer appear to be derived from a special investment of the notochord, and to have penetrated through the membrana elastica externa to reach their final situation. The layer with these cells soon increases in thickness, and forms a continuous unsegmented tube of fibrous tissue with flattened concentrically arranged nuclei ([fig. 317], Vb). Externally is placed the membrana elastica externa (mel), while within is the cuticular sheath of the notochord. This tube is the cartilaginous tube spoken of above and is known as the cartilaginous sheath of the notochord.
Fig. 316. Longitudinal section through a small part of the notochord and adjoining parts of a Scyllium embryo, at the time of the first formation of the cartilaginous sheath.
ch. notochord; sc. sheath of notochord; n. nuclei of cartilaginous sheath; me.e. membrana elastica externa.
The exact origin of the cartilaginous tube just described is a question of fundamental importance with reference to the origin of the vertebral column and the homologies of its constituent parts; but is by no means easy to settle. In the account of the subject in my memoir on Elasmobranch Fishes I held with Gegenbaur that it arose from a layer of cells outside the sheath of the notochord, on the exterior of which the membrana elastica externa was subsequently formed. To this view Götte (No. [419]) also gave his adhesion. Schneider has since (No. [429]) stated that this is not the case, but that, as described above, the membrana elastica externa is formed before the layer of cartilage. I have since worked over this subject again, and am on the whole inclined to adopt Schneider’s correction.
It follows from the above description that the cartilaginous tube in question is an essential part of the sheath of the notochord, and that it is to some extent homologous with the notochordal sheath of the Sturgeon and the Lamprey, and not an entirely new formation.
