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Show 1886.] STRUCTURE OF THE OVUM IN THE 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 through the yolk in any given section, the smaller 42 ; hence the proportion between the two is as 3 : 2. I a m 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. O n the hypothesis that none of the cells contained in the yolk during later stages are derived from the migration inwards of follicular 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 m e 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 m y calculations1 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 3 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 hypothesis 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. A m o n g those 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 characterized 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; these 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 degenerated, evacuating their contents into the interior, or must have 1 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. |
