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 Iblhcular 

 cells and the ovum is necessary to convey adequate nourishment. 



In the stage represented in Plate XXVIII. fig. 3, which is charac- 

 terized 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 XXIX. 

 fig. 7) of the follicular cells, which, however, 1 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 

 XXIX. fig. 6) ; this membrane probably corresponds, in some ova at 

 least, to the vitelline membrane which has persisted after the disap- 

 pearance of the internal zona radiata. In some ova belonging to this 

 stage, the occurrence of a few scattered cells through the substance of 

 the 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 )iew 

 formation, though perhaps still homologous with the vitelline mem- 

 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 obser- 

 vations on Bombinator ' 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 Iwakawa - 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, 



^ Entwickelungsgeschichte der Unke, p. 19. 



2 Quart. Journ. Micr. Sci. vol. xxii, (1882), p. 274, pi. xxiv. figs. 2, 4-26. 



