510 MR. F. E. BKDDARD ON THE DEVELOPMENT AND [DcC. 7, 



the follicular cells, but serves at any rate to determine accurately the 

 boundary line between the follicular layer proper and the mass of 

 peripheral cells which ultimately bear a resemblance to the secondary 

 follicle laver. This limiting band of structureless substance is only 

 met with on that side of the cell-mass towards the outside of the 

 ovary ; elsewhere the cells of the follicular epithelium are perfectly 

 continuous with the cells of the peripheral layer. 



The nature of the cells of the follicular layer also differs from that of 

 the more peripherally placed cells. The cells themselves are rather 

 larger and irregularly rounded in form ; there is no trace of the con- 

 nective-tissue-like structure described above, in the case of the extra- 

 follicular cells. The cell-contents are clear and for the most part 

 hardly affected by the staining-agent, which has coloured their nuclei 

 deeply ; the part of the cell-protoplasm that is coloured is tinged very 

 faintly and shows a reticulate arrangement. The nuclei of the folli- 

 cular cells differ for the most part from those of the extra-follicular 

 layer by being rounded and even in shape, and all closely similar in 

 size ; they are deeply stained, and show a tendency to the same 

 reticulate arrangement of the nuclear substance that has already been 

 mentioned in the estra-foliicular cells. 



The character of this follicular layer is much the same through- 

 out, only differing in places by the more or less crowded condition 

 of the nuclei, indicating a more or less active multiplication of the 

 cells. On that side of the cellular mass which is furthest from the 

 outside of the ovary, the follicular layer comes into closer relations 

 with the extra-follicular epithelial layer, though still recognizable. 

 The character of the cells and of the nuclei which make up the extra- 

 folhcular coat of cells alters, and every transitional condition is met 

 with between these cells and the cells of the foUicular epithelium. 

 This seems to indicate that the folhcular layer is formed as a 

 differentiation of the mass of invaginated germiual cells. 



The whole body is thus surrounded by three distinct and inde- 

 pendent layers — (I) the single layer of large follicular cells ; (2) a 

 vascular layer, to which reference has already been made and which is 

 extremely "developed ; (3) an outermost cellular layer, consisting of 

 flattened cells with nuclei elongated in the ilirection of the circum- 

 ference of the ovum ; this layer, like the follicular layer, is only one 

 cell thick. 



For the most part this outer layer has been neglected by writers, 

 or else has been confused with the true follicular layer. Balfour, 

 however, has recognized it in the Elasmobranch ovum ^ and has 

 proposed to call it the "secondary follicle-layer." Owsiamiikow 

 figures this layer in the ovum of the Perch ^ ; in the explanation of 

 the figure it is called the follicular layer, while the true follicular cells 

 are termed the "granulosa" ; in the text of his paper, however, 

 the term endothelium is constantly used for this layer, which is 

 stated to be made up of several rows of cells in many fishes. 

 Concerning the origin of this layer Balfour expresses with hesitation 



1 Quart. Journ. Micr. Sci. vol. xviii. (1878), p. 405, pi. xis. fig. 29, fe'. 

 ^ Mem. Acad. Sci. St. Petersbourg, t. xxiiii. (]fc£ii) uo. 4, pi. i. fig. 4, a. 



