The third type is characteristic of Lepidosteus, Teleostei, and Petromyzon. Here the axial plate becomes narrowed in such a way that it forms a solid keel-like projection towards the ventral surface ([fig. 191] Me). This keel subsequently becomes separated from the remainder of the epidermis, and a central canal is afterwards developed in it. Calberla and Scott hold that the epidermic layer of the skin is involuted into this keel in Petromyzon, and Calberla maintains the same view for Teleostei ([fig. 32]), but further observations on this subject are required. In the Teleostei a very shallow depression along the axis of the keel is the only indication of the medullary groove of other forms.
In Amphioxus ([fig. 190]), the Tunicata, Petromyzon (?), Elasmobranchii ([fig. 182]), the Urodela and Mammalia ([fig. 187]), the epiblast of the medullary plate is only formed of a single row of cells at the time when the formation of the central nervous system commences; but, except in Amphioxus and the Tunicata, it becomes several cells deep before the completion of the process. In other types the epiblast is several cells deep even before the differentiation of a medullary plate. In the Anura, the nervous layer of the epidermis alone is thickened in the formation of the central nervous system ([fig. 72]); and after the closure of the medullary canal, the epidermic layer fuses for a period with the nervous layer, though on the subsequent formation of the central epithelium of the nervous canal, there can be little doubt that it becomes again distinct.
Fig. 191. Section through an embryo of Lepidosteus on the fifth day after impregnation.
MC. medullary cord; Ep. epiblast; Me. mesoblast; hy. hypoblast; Ch. notochord.
It seems almost certain that the formation of the central nervous system from a solid keel-like thickening of the epidermis is a derived and secondary mode; and that the folding of the medullary plate into a canal is primitive. Apart from its greater frequency the latter mode of formation of the central nervous system is shewn to be the primitive type by the fact that it offers a simple explanation of the presence of the central canal of the nervous system; while the existence of such a canal cannot easily be explained on the assumption that the central nervous system was originally developed as a keel-like thickening of the epiblast.
It is remarkable that the primitive medullary plate rarely exhibits any indication of being formed of two symmetrical halves. Such indications are, however, found in the Amphibia ([fig. 192] and [fig. 72]); and, since in the adult state the nervous cord exhibits nearly as distinct traces of being formed of two united strands as does the ventral nerve-cord of many Chætopods, it is quite possible that the structure of the medullary plate in Amphibia may be more primitive than that in other types[99].
Fig. 192. Transverse section through the cephalic region of a young Newt embryo. (After Scott and Osborn.)
In.hy. invaginated hypoblast, the dorsal part of which will form the notochord; ep. epiblast of neural plate; sp. splanchnopleure; al. alimentary tract; yk. and Y.hy. yolk-cells.