In Birds[206] the horizontal splitting of the mesoblast into somatic and splanchnic layers appears, as in Mammals, to extend at first to the summit of the protovertebræ, but these bodies become so early separated from the parietal plates that this fact has usually been overlooked or denied; but even on the second day of incubation the outer layer of the protovertebræ is continuous with the somatic layer of the lateral plates, and the inner layer and kernel of the protovertebræ with the splanchnic layer of the lateral plates[207]. After the isolation of the protovertebræ the primitive position of the split which separated their somatic and splanchnic layers becomes obscured, but when on the third day the muscle-plates are formed they are found to be _constituted of two layers, an inner and an outer, which enclose between them a central cavity_. This remarkable fact, which has not received much attention, though noticeable in most figures, receives a simple explanation as a surviving rudiment on Darwinian principles. The central cavity of the muscle-plate is, in fact, a remnant of the vertebral extension of the body-cavity, and is the same cavity as that found in the muscle-plates of Elasmobranchii. The two layers of the muscle-plate also correspond with the two layers present in Elasmobranchii, the one belonging to the somatic, the other to the splanchnic layer of mesoblast. The remainder of the protovertebræ internal to the muscle-plates is very large in Birds, and is the equivalent of that portion of the protovertebræ which in Elasmobranchii is split off to form the vertebral bodies[208] (Pl. 11, figs. 6, 7, 8, _Vr_). Thus, though the history of the development of the mesoblast is not precisely the same for Birds as for Elasmobranchii, yet the differences between the two groups are of such a character as to prove in a striking manner that the Avian development is a derivation from a more primary form, like that of the Elasmobranchii. Footnote 206: For the history of protovertebræ and muscle-plates in Birds, vide _Elements of Embryology_, Foster and Balfour. The statement there made that the horizontal splitting of the mesoblast does not extend to the summit of the vertebral plate, must however be regarded as doubtful. Footnote 207: Vide _Elements of Embryology_, p. 56. Footnote 208: Dr Götte, _Entwicklungsgeschichte der Unke_, p. 534, gives a different account of the development of the protovertebræ from that in the text. He states that the muscle-plates do not give rise to the main dorso-lateral muscles, but only to some superficial ventral muscles, while the dorso-lateral muscles are according to him formed from part of the kernel of the protovertebræ internal to the muscle-plates. The account given in the text is the result of my own investigations, and accords precisely with the recent statements of Professor Kölliker, _Entwicklungsgeschichte_, 1876.