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 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, 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 
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 new 
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, Gdtte’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 ie structure of the membrane, 
1 Entwickelungsgeschichte der Unke, p p- 
* Quart. Journ, Mier. Sci. vol. xxii. ‘iBs2), p. 274, pl. xxiv. figs. 2, 4-26. 
