1886.] STRUCTURK OF THis; OVUM IN THK DIPNOI, 517 



than twice the size of the smaller body selected for comparison ; the 

 number of cells in the interior was in correspondence with their 

 difference in size. The larger had an average of 64 cells disseminated 

 tiirough the yolk in any given section, the smaller 42 ; hence the 

 proportion between the two is as 3 : 2. 



I am inclined to lay all the more stress on the mathematical 

 statement of the case, as it allows extremely wide limits for possible 

 errors of computation. 



On the hypothesis that none of the cells contained in the yolk 

 during later stages are derived from the migration inwards of follicu- 

 lar cells, it is obviously necessary to assume that they are all produced 

 by the division of the central cells, or by certain of these cells which 

 have persisted without division. It is true that the nuclei of the 

 central cells in the earlier stages do multiply, but it is equally certain 

 that others degenerate and disappear ; and it seems to me that more 

 undergo the latter than the former change ; and I find that in the 

 stage referred to above the smaller body contains considerably fewer 

 cells in any given transverse section than in Stage I. It would then 

 be expected that the larger bodies would contain fewer and fewer 

 cells in their interior. I have, however, just stated that the result 

 of my calculations ' in this respect has been in the direction of proving 

 an increased number of cells in the more mature bodies. Now, 

 assuming that I have made so large an error as j of the total 

 number of cells in the larger, the two would still contain an equal 

 number of cells disseminated through the yolk. But on the hypo- 

 thesis there ought to be a very considerably less number of cells in 

 the larger body. It is clear therefore that this hypothesis cannot 

 be maintained ; and as there is no ground for assuming any third 

 origin of the cells, it seems most probable that they have been 

 largely derived from the proliferation of the follicular layer. 



Among tliose which I have included in this same stage are many 

 that are probably, owing to their smaller size, younger than others 

 which are larger. I have not, however, thought it worth while to 

 separate these into two distinct stages, since they are both charac- 

 terized by the extraordinary activity of the follicular epithelium, and 

 by the presence of masses of yolk in the interior of the follicle, in 

 which are imbedded numerous cells, themselves filled with yolk- 

 spherules. As a general rule the smaller bodies belonging to this 

 stage can be distinguished from the larger by the more crowded 

 follicular cells ; the^^e are smaller, placed closer together, and not 

 confined to a single layer in the smaller, and therefore less mature, 

 specimens ; in the larger bodies these cells have increased in size, 

 the nuclei are not so crowded together, and the cells form but a 

 single layer. This condition can hardly have been arrived at by the 

 mere mechanical growth in size of the whole body, which would tend 

 to stretch, and therefore to reduce to a single layer of cells, the 

 follicular epithelium ; the cells themselves must either have dege- 

 nerated, evacuating their contents into the interior, or must have 



' I have also calculated the number of cells contained in the interior of two 

 other pairs of ova belonging to this stage, and of about the same relative size. 



