The section figured, whose features I am describing, belongs to the middle of the dorsal region. Anteriorly to this point the spinal cord becomes more elliptical in section, and the spinal canal more lanceolate; posteriorly, on the other hand, the spinal canal and tube become more nearly circular in section. Immediately beneath the neural tube is situated the notochord (_ch_). It exhibits at this stage a central area rich in protoplasm, and a peripheral layer very poor in protoplasm; externally it is invested by a distinct cuticular membrane. Beneath the notochord is a peculiar rod of cells, constricted from the top of the alimentary canal[48]. On each side and below this are the two aortæ, just commencing to be formed, and ventral to these is the alimentary canal. Footnote 48: Vide Balfour, "Preliminary account of the Development of Elasmobranch Fishes," _Quart. Journ. of Microsc. Science_, Oct. 1874, p. 33. [This edition, p. 96.] On each side of the body two muscle-plates are situated; their upper ends reach about one-third of the way up the sides of the neural tube. The two layers which together constitute the muscle-plates are at this stage perfectly continuous with the somatic and splanchnic layers of the mesoblast, and the space between the two layers is continuous with the body-cavity. In addition to the muscle-plates and their ventral continuations, there are no other mesoblast-cells to be seen. The absence of all mesoblastic cells dorsal to the superior extremities of the muscles is deserving of special notice. Very shortly after this period and, as a rule, before a third visceral cleft has become visible, the first traces of the spinal nerves make their appearance. _First Stage._--The spinal nerves do not appear at the same time along the whole length of the spinal canal, but are formed first of all in the neck and subsequently at successive points posterior to this. Their mode of formation will be most easily understood by referring to Pl. 22, figs. B I, B II, B III, which are representations of three sections taken from the same embryo. B I is from the region of the heart; B II belongs to a part of the body posterior to this, and B III to a still posterior region. In most points the sections scarcely differ from Pl. 22, fig. A, which, indeed, might very well be a posterior section of the embryo to which these three sections belong. The chief point, in addition to the formation of the spinal nerves, which shews the greater age of the embryo from which the sections were taken is the complete formation of the aortæ. The upper ends of the muscle-plates have grown no further round the neural canal than in fig. A, and no scattered mesoblastic connective-tissue cells are visible.