1886.] | STRUCTURE OF THE OVUM IN THE DIPNOI. 519 
problematical structures will be described in this portion of my paper, 
although they do not appear to belong to the same series as that 
which I shall now describe. 
The body (fig. 25) is decidedly smaller than in Protopterus; it 
corresponds, however, very nearly to the stage illustrated in fig. 9 of 
Plate LIII., and which is fully described on p. 513, where the slight 
differences which it presents from other individuals belonging to 
Stage I. are pointed out. The correspondence is in structure as 
well as in size. 
The body is placed at some little distance from the external 
surface of the ovary, but is connected with the germinal epithelium 
of the surface of the ovary by a slender pedicle of cells; it con- 
sists, like the corresponding structure of Protopterus, of a hollow 
sphere of cells which enclose a central mass ; the peripheral and 
central cells are also more or less independent. The peripheral 
layer of cells forms a continuous whole, but a closer examination 
shows it to be made up of two layers which are occasionally very 
distinctly separable from each other. The outermost layer appears 
to have originated from the germinal epithelium; its nuclei are 
large, rounded, or oval, and closely pressed together; the cells 
themselves, which contain these nuclei, have for the most part a 
stroma-like appearance (see p. 508). Within this layer, which is 
often several cells thick, is a layer of blood-vessels ensheathed in a 
mass of tissue of a fibrous retiform character, the fibres (fig. 25, a) 
for the most part forming a layer running parallel with the cireum- 
ference of the ovum, with interspersed nuclei; this tissue, from its 
general appearance and from its intimate connection with the blood- 
cavities (fig. 25, 41), is probably derived, like the blood-vessels, from 
ingrowths of stroma. The nuclei of this presumed stroma-layer are 
on the whole more elongated in form than those of the outer layer ; 
the blood-vessels were gorged with blood. 
The central mass of cells is probably during life in contact with the 
peripheral layers, but it appeared to be for the greater part at least 
quite distinct from it, there being no transition between its cells and 
those of the peripheral layers. The central cells appear to be closely 
similar in structure to the corresponding cells in Protopterus; the 
protoplasm of the cell is reticulate with large spaces left between 
the individual strands which form the network; the nucleus is of 
moderate size, round or oval in shape. Here and there (figs. 25, 26, 
f.e) some of the central cells were disposed in a row, one cell thick, 
round the periphery, closely applied to the innermost (stroma) layer 
of the peripheral layers. This is possibly to be compared to the 
true follicle-layer in Protopterus. 
Although, as I have already stated, the material at my disposal 
in the case of Ceratodus was not well preserved, certain portions of 
the ovary were in a better condition than others, and, generally 
speaking, it was quite possible to make out the relations of the 
different parts of the organ, both the stroma and contained ova, as 
well as occasionally the germinal epithelium on the outside; the 
minutiee of structure of the different cells were disguised by the inferior 
