548 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
section of the body may cut through two or more of these sexual 
glands. 
In the young gonad all of the cells seem to be exactly alike in 
size and general appearance, but they soon exhibit variations both in 
size and in structure, so that three types of cells are recognizable 
(pi. 23, figs. 7, 8), young ova, yolk cells, and follicular cells. 
The follicular cells are flattened and form a thin layer within the 
wall of the gonad, constituting an investment for the egg when 
nearly mature; they also furnish the cells which form the duct 
through which the sexual gland is connected with the exterior of 
the body. The follicular cells at the side of the gonad nearest 
the body wall become somewhat spheroidal in form and collected 
into a small cluster. They then grow outward into the overlying 
muscular layers of the body wall to form a sharply pointed duct> 
lined with flattened cells, which projects outward as far as the base¬ 
ment layer of the integument (as shown for the spermary, pi. 25, 
fig. 26). The duct is formed to this extent at a very early period 
in the development of the ova, and remains in this condition until 
the single ovum developing in the gonad becomes well advanced 
toward maturity, when it completes its growth to the exterior, 
opening in all cases dorsal to the lateral nerve. 
The yolk cells (ov r .) are apparently merely egg cells which for 
some reason become aborted at an early period. They are rather 
numerous in each young ovary, and grow to a size several times that 
of the largest follicular cells. They become crowded with yolk 
granules and pressed closely against the wall of the developing 
ovum. To all appearances the cell membranes separating ovum and 
yolk cells break down and the latter are absorbed directly into the 
substance of the developing ovum. The wdiole substance—cyto¬ 
plasm, yolk, and nucleus — of the yolk cell seems to be thus 
absorbed by the ovum. 
There may be several large cells in the young ovary which are 
distinguished by a more homogeneous cytoplasm from the yolk cells 
with which they are surrounded. These cells increase rapidly in 
size and are recognizable as young ova. Eventually, however, all 
save one, or rarely two, of these ova cease their growth and become 
disintegrated to furnish food materials for the surviving cell. The 
mature ovary therefore usually contains but a single large ovum 
(pi. 23, fig. 8). 
