Shortly after the floor of cells has appeared, the whole segmentation cavity becomes obliterated. When the embryonic rim has attained to some importance, the position of the embryo becomes marked out by the appearance of the medullary groove at its most projecting part. The embryo extends from the edge of the blastoderm inwards towards the centre. At about the time of the formation of the medullary groove, the mesoblast becomes definitely constituted. It arises as two independent plates, one on each side of the medullary groove, and is entirely derived from lower layer cells. The two plates of mesoblast are at first unconnected with any other cells of the blastoderm, and, on their formation, the hypoblast remains in connection with all the remaining lower layer cells. Between the embryonic rim and the yolk is a cavity,--the primitive alimentary cavity. Its roof is formed of hypoblast, and its floor of yolk. Its external opening is homologous with the anus of Rusconi, of Amphioxus and the Amphibians. The ventral wall of the alimentary cavity is eventually derived from cells formed in the yolk around the nuclei which are there present. * * * * * Since the important researches of Gegenbaur[149] upon the meroblastic vertebrate eggs, it has been generally admitted that the ovum of every vertebrate, however complicated may be its apparent constitution, is nevertheless to be regarded as a simple cell. This view is, indeed, opposed by His[150] and to a very modified extent by Waldeyer[151], and has recently been attacked from an entirely new standpoint by Götte[152]; but, to my mind, the objections of these authors do not upset the well founded conclusions of previous observations. Footnote 149: "Wirbelthiereier mit partieller Dottertheilung." Müller's _Arch._ 1861. Footnote 150: _Erste Anlage des Wirbelthierleibes._ Footnote 151: _Eierstock u. Ei._ Footnote 152: _Entwicklungsgeschichte der Unke._ The important researches of Götte on the development of the ovum, though meriting the most careful attention, do not admit of discussion in this place. As soon as the fact is recognised that both meroblastic and holoblastic eggs have the same fundamental constitution, the admission follows, naturally, though not necessarily, that the eggs belonging to these two classes differ solely in degree, not only as regards their constitution, but also as regards the manner in which they become respectively converted into the embryo. As might have been anticipated, this view has gained a wide acceptance.