SEX DETERMINATION IN DINOPH1LUS GYROC1LIATUS. 355 
periments before any results can be attained. It is, however, 
a comparatively easy matter to get them to live to the stage 
represented in the above text-figure, and from this it is clear 
that the sperm part of tlie nucleus is missing. Comparison 
of Text-fig. 5b with fig. 24 brings out this point clearly. The 
stage, however, represented in this text-figure is much earlier 
than that at which any possible sex determination can take 
place in the unfertilised female. 
This experiment confirms the nature of the small accessory 
nucleus of the fertilised oogonial cell, and shows that it is 
spermatic in origin. The fact also brought out by this ex- 
periment, that when the sperm are prevented from entering 
the obgonial cells, these grow slowly, and apparently take 
months to reach the stage they would normally attain in a few 
days under the influence of the sperm, shows the sperm to 
play an important rol e in the metabolism of the oogonial cells, 
that may even extend to the growth of the female in general. 
It is at least remarkable that many of the females in which 
early fertilisation has been prevented never attain anything 
like the size of the fertilised ones. 
As I have said, the primitive oogonial cells seem to be 
dividing most actively in the anterior region of the ovary, 
while in the posterior region they seem to be increasing in 
size. The multiplication of the oogonial cells is so rapid, 
however, that the whole organ becomes crowded with them 
before those of the posterior portion, which are the oldest, 
have had time to grow very much (fig. 9). The result of this 
is that there is a stage in which all the oogonial cells are much 
the same in appearance and of approximately the same size. 
Each has been joined by a spermatazoon, the head of which 
has lodged in the cytoplasm (fig. 24). This stage is followed 
by one in which they increase in size, but as far as I can 
determine do not actively divide. When they have increased 
several times in size, their nuclei undergo a senes of rapid 
divisions, not followed by any division of their cytoplasm. It 
is possible that these divisions are, at the same time, accom- 
panied by some fusing together of the cells. At this point it 
