1886.] STRUCTURE OF THE OVUM IN THE DIPNOI. 515 
greatly diminished ; it forms only a single layer of cells, and the 
pedicle which connects the body with it is almost entirely composed 
of stroma-ingrowths, which form for the most part a very lax net- 
work of fibres and cells, though denser in the middle, and sur- 
rounding a slender cord of germinal cells, the sole remnant of the 
former epithelial pedicle. 
In a somewhat more advanced stage there is an identical structure 
with that just described as regards the specialization of the peri- 
pheral layers and the mass of central cells; the formation of yolk 
has, however, gone on much faster, and the follicular cells, as well as 
most of the cells of the central mass, are crammed with variously 
sized yolk-spherules. 
Here and there, especially in the periphery of the central cells, are 
irregularly shaped masses of yolk-spherules (fig. 24, a), among which 
are occasionally discernible nuclei like those of the surrounding cells. 
A comparison of Plate LIV. fig. 24 with Plate LITT. fig. 11 will show 
how very similar in size and extent these irregular patches of yolk 
are to the patches of amorphous deeply staining protoplasm in the 
earlier stage; and I cannot but think that they are these same 
patches of protoplasm produced by the fusion of some of the central 
cells which have commenced to form yolk-particles on their own 
account. 
Other preparations, again, seem to indicate that the fusion of 
certain of the central cells either does not take place at all, or takes 
place after the formation of the yolk. I have a nearly complete 
series of sections through an ovum in which the yolk has already 
commenced to be formed; the cells of the follicular epithelium are 
full of yolk-particles, as are also the central cells. The latter form 
a compact mass of cells containing abundant yolk-particles, but 
without any definite patches of yolk lying between the cells, such as 
could be compared to the patches of protoplasm formed by a fusion 
of cells. In many sections, however, of this series it happened that 
the central mass of cells had dropped out, so that I cannot make any 
positive statements about the point of discussion raised. This par- 
ticular instance showed very plainly indeed the connection of the 
mass of central cells with the follicular epithelium ; the area of con- 
nection was very limited, as has been already mentioned in other 
cases. 
The central cells are united with the follicular epithelium by a 
bridge of tissue which has every appearance of being an outgrowth 
of the latter; the nuclei are round and pressed close together as if 
in a condition of active multiplication ; they pass without avy break 
into the follicular epithelium, from the cells of which they cannot, 
indeed, be distinguished, and gradually on the other side into the 
mass of central cells. The cells of the latter have the ordinary 
characters that have been already described. I noticed a very large 
number of peculiar cells, several of which are illustrated in fig. 4 ; 
similar cells are not absent in other cases, but I never have seen 
them so numerous as in the present case. These cells are easily 
to be distinguished from the germinal cells among which they lie 
34* 
