1885. | STRUCTURE OF THE OVUM IN THE DIPNOI. 511 
the opinion that it may be derived from the germinal epithelium. 
Owsiannikow suggests three possibilities either it originates from 
cells which have made their way out of the blood-vessels (!), or from 
cells of the subgerminal tissue (stroma ?), or, finally, they may be 
derived from the germinal layer. The latter alternative is adopted 
by Owsiannikow on certain evidence, which he does not, however, 
regard as conclusive. The outermost follicular layer of Lepidosiren 
I have already (p. 509) shown without doubt to be derived from 
the germinal epithelium; I shall therefore adopt the name of 
secondary follicular epithelium for this capsule, which indicates that 
its origin is similar to that of the true follicular epithelium, which 
may be briefly termed the follicular epithelium. 
The two layers that have just been described form a hollow sphere 
enclosing a central cavity, which is partly occupied by a mass of 
cells. It is very possible that in the fresh condition the central 
mass of cells occupies the whole of the space available, but this 
is not the case in my preparation. A large portion of the central 
cavity, particularly on the side turned towards the exterior of 
the ovary, is quite empty, and no structures intervene between 
the central mass of cells and the follicular layer. On the opposite 
side, however, the central mass is in close contact for a con- 
siderable area with the follicular cells, this area exactly corre- 
sponding with the transitional area between the follicular and external 
layers. These facts would suggest that the central cells are derived 
from the proliferation of the follicular cells and ultimately of the 
extra-follicular cells, as these two latter have been shown to be 
perfectly continuous, the proliferation taking place in a certain 
limited area only. In this case the apparent cavity which separates 
the central cells from the follicular on one side will be an indication 
(exaggerated by the action of the preservative reagent) that there is 
here no real connection between the central and peripheral layers, 
though they may be in actual contact in the fresh state. 
A number of the central cells are displayed in figs. 14—20 of Plate 
LIII.; they are more or less irregular in shape, rounded, and of 
different sizes; the staining-reagent has hardly affected the cell- 
protoplasm, but has deeply stained the nucleus. The cell-protoplasm 
is arranged in a reticulate fashion, and closely resembles that of the 
follicular cells. Some of the cells contain two or more nuclei, which 
seems to show that the cells themselves are in a condition of multi- 
plication. The most remarkable fact about the nuclei of the central 
plug of cells is their great inequality in size: some of the variations 
are exhibited in those figures ; the variation is all the more remarkable 
as it does not occur in the follicular layer, the nuclei of whose cells 
are of quite a uniform size. There is almost every gradation in size 
between the smallest and largest nuclei, a fact which perhaps indicates 
that the smaller ones are the result of nuclear division. The largest 
nuclei rather excel in size those of the follicular epithelium. There 
is a similar difference of size in the peripberal layer of cells, 
particularly obvious at those points where the peripheral layer is in 
1 Loe, cit. p. 30. 
