SEX DETERMINATION IN D1NOPHILUS GYROCILIATUS. 333 
cells first begin to appear the female has grown very much, 
and since hatching' has increased ten or fifteen times in size. 
The germ-cells themselves are first distinguishable as very 
minute refractive nuclei surrounded with a small amount of 
cytoplasm in the upper part of the mass of transparent tissue 
already mentioned, this mass of tissue being in short a 
primitive ovary. I have been able to determine very little 
regarding the exact origin of the germ-cells beyond the fact 
that they do not arise from the endoderm cells of the gut as 
Yon Malsen (8) has sug-gested. The ovary is surrounded by 
a firm membrane, which is shown at a later stage in Text- 
fig. 3, and it is necessary to assume that if the primitive 
germ-cells arise from the gut wall, then they have to pass 
through this membrane in order to take up their final position. 
Yon Malsen (8), however, was unaware of the presence of this 
membrane, which has only recently been demonstrated by 
Nelson (11) in Dinopliilus conklinii. Moreover, the 
germ-cells are never seen arising from the gut wall, but are 
always within this membrane. 
Once the female germ-cells have appeared their subsequent 
growth and rapid multiplication can be readily studied and 
their relationship to the sperm injected into the ovary at the 
time of hatching watched. It may perhaps add to the 
clearer understanding of the following paper if I briefly 
outline what I think takes place, and then describe the 
various stages of the process in detail later. 
To recapitulate, then, briefly, the two varieties of eggs, 
male and female, are normally laid together in one capsule, 
but they develop immediately without the presence of the male, 
and are therefore fertilised inside the body of the female, or 
else they develop parthenogenetically. That they are fertilised 
in the female the subsequent observations show. In a few 
days the small male egg gives rise to the rudimentary male, 
which is full grown and sexually mature (Text-fig. 1) at the 
time the females are ready to leave the capsule. 
The female, on the contrary, when she leaves the capsule, is 
very small and still in the larval state (figs. G, 5, 4), the cilia- 
