Footnote 111: "Recherches sur la segmentation des products adventifs de l'oeuf des Plagiostomes et particulièrement des Raies." Robin, _Journal de l'Anatomie et de la Physiologie_, p. 609, 1872. Schenk[112] has also made important investigations on the subject. He considers that the ovum is invested with a very delicate membrane. This membrane I have failed to find a trace of, and agree with Leydig[113] in denying its existence. Schenk further found that after impregnation, but before segmentation, the germinal disc divided itself into two layers, an upper and a lower. Between the two a cavity made its appearance which Schenk looks upon as the segmentation cavity. Segmentation commences in the upper of the two layers, but Schenk does not give a precise account of the fate of the lower. I have had no opportunity of investigating the impregnated ovum before the commencement of segmentation, but my observations upon the early stages of this process render it clear that no division of the germinal disc exists subsequently to the commencement of segmentation, and that the cavity discovered by Schenk can have no connection whatever with the segmentation cavity. I am indeed inclined to look upon this cavity as an artificial product. I have myself met with somewhat similar appearances, after the completion of segmentation, which were caused by the non-penetration of my hardening reagent beyond a certain point. Footnote 112: "Die Eier von Raja quadrimaculata innerhalb der Eileiter." _Sitz. der k. Akad. Wien._ Vol. LXXIII. 1873. Footnote 113: _Loc. cit._ My denial of the existence of this membrane naturally applies only to the egg after impregnation, and to the genera Scyllium and Pristiurus. Without attempting absolutely to explain the appearances described by Professor Schenk, I think that his observations ought to be repeated, either by himself or some other competent observer. Several further facts are recorded by Professor Schenk in his interesting paper. He states that immediately after impregnation, the germinal disc presents towards the yolk a strongly convex surface, and that at a later period, but still before the commencement of segmentation, this becomes flattened out. He has further detected amoeboid movements in the disc at the same period. As to the changes of the germinal disc during segmentation, his paper contains no facts of importance. Next in point of time to the paper of Schenk, is my own preliminary account of the development of the Elasmobranch Fishes[114]. In this a large number of the facts here described in full are briefly alluded to. Footnote 114: _Loc. cit._ The last author who has investigated the segmentation in Elasmobranchii, is Dr Alexander Schultz[115]. He merely states that he has observed the segmentation, and confirms Professor Schenk's statements about the amoeboid movements of the germinal disc.