Footnote 201: _Grundriss d. Vergleichenden Anat._ p. 494. Dr Dohrn has not as yet given the grounds for his determination, so that any judgment on his views is premature. None of my observations on Elasmobranchii lends any support to these views; but perhaps, while regarding the limbs as the remains of a continuous fin, it might be permissible to suppose that the pelvic and thoracic girdles are altered remnants of the skeletal parts of some of the gill-arches which have vanished in existing Vertebrates. The absence of limbs in the Marsipobranchii and Amphioxus, for reasons already insisted upon by Dr Dohrn[202], cannot be used as an argument against limbs having existed in still more primitive Vertebrates. Footnote 202: _Loc. cit._ Though it does not seem probable that a dorsal and ventral fin can have existed contemporaneously with lateral fins (at least not as continuous fins), yet, judging from such forms as the Rays, there is no reason why small balancing dorsal and caudal fins should not have co-existed with fully developed lateral fins. _Mesoblast. G-K._ The mesoblast in stage F forms two independent lateral plates, each with a splanchnic and somatic layer, and divided, as before explained, into a vertebral portion and a parietal portion. At their peripheral edge these plates are continuous with the general mesoblastic tissue of the non-embryonic part of the blastoderm; except in the free parts of the embryo, where they are necessarily separated from the mesoblast of the yolk-sac, and form completely independent lateral masses of cells. During the stages G and H, the two layers of which the mesoblast is composed cease to be in contact, and leave between them a space which constitutes the commencement of the body-cavity (Pl. 10, fig. 1). From the very first this cavity is more or less clearly divided into two distinct parts; one of them in the vertebral portion of the plates of mesoblast, the other in the parietal. The cavity in the parietal part of the plates alone becomes the true body-cavity. It extends uninterruptedly through the anterior parts of the embryo, but does not appear in the caudal region, being there indicated only by the presence of two layers in the mesoblast plates. Though fairly wide below, it narrows dorsally before becoming continuous with the cavity in the vertebral plates. The line of junction of the vertebral and parietal plates is a little ventral to the dorsal summit of the alimentary canal (Pl. 10, fig. 5). Owing to the fact that the vertebral plates are split up into a series of segments (protovertebræ), the section of the body-cavity they enclose is necessarily also divided into a series of segments, one for each protovertebra.