Footnote 209: The type of development of the muscle-plates of Amphibians would become identical with that of Elasmobranchii if their first-formed mass of muscle corresponded with the early-formed muscles of Elasmobranchii, and the remaining cells of both layers of the protovertebræ became in the course of development converted into muscle-cells indistinguishable from those formed at first. Is it possible that, owing to the distinctness of the first-formed mass of muscle, Dr Götte can have overlooked the fact that its subsequent growth is carried on at the expense of the adjacent cells of the somatic layer? Osseous Fishes are stated to agree with Amphibians in the development of their protovertebræ and muscular system[210], but further observations on this point are required. Footnote 210: Ehrlich, "Ueber den peripher. Theil d. Urwirbel." _Archiv f. Mic. Anat._ Vol. XI. Though the development of the general muscular system and muscle-plates does not, according to existing statements, take place on quite the same type throughout the Vertebrate subkingdom, yet the comparison which has been instituted between Elasmobranchii and other Vertebrates appears to prove that there are one or two common features in their development, which may be regarded as primitive, and as having been inherited from the ancestors of Vertebrates. These features are (1) The extension of the body-cavity into the vertebral plates, and subsequent enclosure of this cavity between the two layers of the muscle-plates; (2) The primitive division of the vertebral plate into a somatic and a splanchnic layer, and the formation of a large part of the voluntary muscular system out of the splanchnic layer. * * * * * The ultimate derivation of the mesoblast forms one of the numerous burning questions of modern embryology, and there are advocates to be found for almost every one of the possible views the question admits of. All who accept the doctrine of descent are agreed that primitively only two embryonic layers were present--the epiblast and the hypoblast--and that the mesoblast subsequently appeared as a distinct layer, after a certain complexity of organization had been attained. The general agreement stops, however, at this point, and the greatest divergence of opinion exists with reference to all further questions which bear on the development of the mesoblast. There appear to be four possibilities as to the origin of this layer. It may be derived: (1) entirely from the epiblast, (2) partly from the epiblast, and partly from the hypoblast, (3) entirely from the hypoblast, (4) or may have no fixed origin. The fourth of these possibilities may for the present be dismissed, since it can be only maintained should it turn out that all the other views are erroneous. The first possibility is supported by the case of the Coelenterata, and we might almost say by that of this group only[211].