188().] 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. 25J 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 mdividuals 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 sleuder 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 circum- 

 ference of the ovum, with interspersed nuclei ; this tissue, from its 

 general appearance and from its intimate connection with the blood- 

 cavities (fig. 25, bl), 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 coutact 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 slia|)e. 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 

 minutise of structure of the different cells were disguised by the inferior 



