1886. ] STRUCTURE OF THE OVUM IN THE DIPNOL. 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 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 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 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 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 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 namercus 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 celis; 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 mevhanical 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 themselyes must either have dege-~ 
nerated, evacuating their contents into the interior, or must have 
1 T 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. 
