| OCR Text |
Show 1886.] OVUM OF LEPIDOSIREN. 275 looked in good sections. The vitelline membrane in earlier stages is so thin that I have found it impossible to detect any pores ; it may be that they are present, or that the nutrition of the ovum in the earlier stages is carried on, as suggested above, by osmosis, while during the later stages, when the formation of yolk is going on and the need for nutrition increased, direct contact between the follicular cells and the ovum is necessary to convey adequate nourishment. In the stage represented in Plate XXVIII. fig. 3, which is characterized by the extraordinary proliferation of the follicular cells and their migration into the interior of the ovum, there was no trace whatever of any membrane. The ovum lies within the follicular epithelium, and in actual contact with its cells. Indeed the very migration of the follicular cells into the ovum would necessitate the absence of such a membrane, and there were, at any rate, no traces of it except in the well-marked limiting membranes (see Plate X X I X. fig. 7) of the follicular cells, which, however, I never observed to be separated from the cells themselves, and were continuous all round them. The absence of any such membrane round the ova of this stage is one of the strongest arguments against regarding them as a stage intercalated between the last and the next to be described. Into this question I shall enter later. In the latest stages, in which the ovum is entirely occupied by yolk, the follicular epithelium is separated from the contents of the ova by an extremely fine and delicate homogeneous membrane (Plate X X I X . fig. 6) ; this membrane probably corresponds, in some ova at least, to the vitelline membrane which has persisted after the disappearance of the internal zona radiata. In some ova belonging to this stage, the occurrence of a few scattered cells through the substance of tbe yolk appears to indicate that they have been derived from ova belonging to the stage just referred to ; in these also a thin delicate membrane lay between the ovum and the surrounding follicular epithelium. In this case the membrane must be regarded as a new formation, though perhaps still homologous with the vitelline m e m brane. In the number and structure of its membranes the ovum of Lepidosiren appears to be related much more nearly to the Elasmo-branchs than to the Amphibia, with which group the general anatomical structure of the reproductive organs more closely corresponds. In the Amphibia in fact, according to Waldeyer and Kolessnikow, there is only a single delicate membrane, developed comparatively late, and showing a radial striation. Gotte's observations on Bombinator l point to the existence of a single membrane, clear and structureless, which arises by a metamorphosis of the external layer of the ovum. In Triton Iwakawa2 describes and figures a single structureless membrane surrounding the ovum. In these cases the single membrane evidently represents the membrane in Lepidosiren which I have termed vitelline. The discrepancies in the observations of these authors on the structure of the membrane, 1 Entwickelungsgeschichte der Unke, p. 19. 2 Quart. Journ. Micr. Sci. vol. xxii. (1882), p. 274, pi. xxiv. figs. 2, 4-26. |
