SEX DETERMINATION IN DINOPHILUS GYROCILIATUS. 359 
shown with its single chromatic body. The nuclei of the 
male and female eggs can be further compared in the early 
stages by the examination of figs. 26 and 23. Here in the 
female egg (fig. 27) the male and female portions of the 
nucleus are still distinguishable (m. 'pro.; fe\ pro.), while 
only one large chromatic body is present in the male egg 
(fig. 26). In fig. 13 these differences are well shown in a 
whole preparation. Here the large female eggs always have 
two conspicuous chromatic bodies, while the male eggs, which 
usually occupy the anterior ends or median posterior portion 
of the ovary, have one. In the lower ventral portion of the 
ovary, amongst the small oogonial cells, are many darkly 
staining sperm heads. It must be remembered that the 
evidence for this process of sex-determination has to be 
pieced together from a number of different sections, for it 
is, of course, impossible to follow these changes on living 
material and actually see the sex-differentiation that follows 
this division of the nucleus. The evidence of sections, as 
far as it goes, seems to be in favour of the view I have put 
forward above. 
There is, however, another possible way of throwing light 
on the matter, and that is by experimental means. By the 
exclusion of the males we should expect that all the eggs 
should be male and of the same size. Here again it must be 
kept in mind that even in the male eggs their cytoplasm has 
been under the influence of the sperm in all the divisions 
previous to the sex-determining one. Their exclusion may 
not result in the parthenogenetic eggs bearing characters 
similar in every respect to the sexual. What experimental 
evidence I have been able to obtain is of a doubtful nature, 
. and for that reason I should like to defer its consideration to 
a future paper. 
5. Maturation Stages in the Male and Female Egg. 
To obtain good sections of the maturation stages many 
technical difficulties have to be overcome ; the large female 
