s 232 N. M. STEVENS, 
during the whole growth period of the oocytes. The nucleoli in this 
case seem without doubt to be excretion or reduction products, and 
not the “Nucleinspeicher” or “Nucleinlaboratorium” of 
Fick (1899). 
Method of fertilization. — The young oocytes, as they in- 
crease in size, move out from the central region of the ovary (Fig. 1b 
and e) and each one becomes connected with two of the epithelial 
cells which are just lateral to that region. One of these accessory 
cells remains attached to the wall of the spermduct, while the other 
gradually comes into a position, between it and the oocyte, and later 
sinks into the surface of the oocyte. The relation of the oocyte to 
its accessory cells in early stages is well shown in Figs. 1b, c, d, e; 
in later stages in Figs. 2, 16, 17, 18. 
The two accessory cells soon increase in size, and are easily 
distinguished from the other epithelial cells by their different staining 
qualities, and by their larger nuclei and usually much larger nucleoli. 
The cell next to the oocyte always stains much more deeply in later 
stages than the other cell. Fig. 17 shows these cells in connection 
with an oocyte of about the age of that shown in Fig. 10. Within 
the outer cell has appeared a flask-shaped cavity with the neck 
penetrating the wall of the spermduct; the other cell is flattened 
against the end of the oocyte and is still separated from it by a distinct 
membrane. In Fig. 18, the development in the outer cell has gone 
somewhat farther; the tube has passed nearly through the spermduct 
wall; and at the other end of the vesicle, a second tube has pene- 
trated the other accessory cell, which is now sunken into the surface 
of the oocyte. At this stage no separating membrane is evident be- 
tween the inner accessory cell and the cytoplasm of the oocyte. 
In Fig. 2b, the tube has extended through the wall of the sperm- 
duct, and a thick membrane has formed over the whole surface of 
the oocyte except the portion covered by the accessory cells, where a 
micropyle-like opening is formed. 
Fig. 16 shows a slightly later stage more highly magnified. Here 
the spermatozoon has made its way from the spermduct through the 
tube c, and the opening in cell « into the tube which extends through 
cell db. This is the latest stage that my material contains. In all 
cases where the spermatozoon has entered the accessory cells, the 
base of the tube in the spermduct wall is much thickened and probably 
closed. This figure also shows the laminated structure of the egg- 
membrane, and its relation to the accessory cells. In some cases the 
