Habits and structure of Cotylaspis insignis Lerpy. 299 
with a large nucleolus, scattered grains of chromatine and a small 
amount of cytoplasm. In the larger cells the cytoplasm is more 
abundant and is beginning to be pervaded with minute droplets of 
yolk. The still larger cells, have a diameter of 0.02 mm (Fig. 59), 
a cell wall is seen and next it a layer of yolk droplets, then a 
hollow space, and then a central mass of protoplasm with a nucleus. 
This is the final form of the yolk cell, in which it passes down to 
and is found in the yolk receptacle. 
The yolk receptacle lies between the ovary and the spermary. 
It is a triangular cavity, two angles are at the junction of the two 
ducts from the vitellaria, the third angle being at the origin of 
the passage leading out to the oviduct. The wall of this organ can 
be traced in living specimens under compression out into the three 
different ducts and by pressure the contents can be forced out 
through the ducts. The wall in sections is seen to be distinctly 
cellular as indicated by the presence of flattened nuclei, unlike that 
of the vitellarian follicles in which nuclei can not be recognized. 
I have not observed any muscular tissue in this wall in sections in 
which it is easily seen in the oviduct. The yolk receptacle is filled 
with yolk-cells identical in appearance with the largest cells in the 
vitellaria, which clearly have merely been collecting and waiting 
here till an egg cell shall descend, when they will join it, and in 
the oviduct near by acquire a shell, thereby completing the egg. 
In many of the trematodes there are two organs at or just beyond 
the junction of the yolk-duct and the oviduct, the ootype in which 
the embryo is formed and the shell gland, a gathering of cells 
surrounding the ootype which produces the substance of which the 
shell is composed. I have not succeeded in recognizing either of 
these as distinct organs in C. insignis; doubtless here the ootype is 
never more than a slightly specialized part of the oviduct. Thus 
in Macraspis JÄGERSKIÖLD (1899) refers to it by name, though in 
his figure no distinct part is represented to correspond. According 
to STAFFORD a distinct enlargement of the oviduct is found just 
beyond the junction of the yolk-duct in Aspidogaster with much 
thicker walls “lined with high epithelium cells” and surrounded by 
unicellular glands which contribute the shell substance. I have not 
been able to find a similar organ in sections of C. insignis. 
The eggs are lodged in swollen places in different parts of the 
uterus beyond the entrance of the yolk-duct. These places are not 
constant in location, and seem to be formed temporarily to accom- 
