SEX DETERMINATION IN DINOFHILUS G YKOCILIATTTS. 335 
male and female portions of the nucleus divide equally, so 
that a similar quantity of nuclear material, both male and 
female, gets into each daughter-nucleus. There are probably 
about forty to fifty divisions in all. In these the male and 
female parts of the nucleus divide and move apart, the male 
portion usually dividing first (fig. 35). Now and again, how- 
ever, the female half of the nucleus seems to divide before 
the male portion, so that the male portion gets left behind 
and is shut off entirely in one of the daughter-nuclei (fig. 33). 
Therefore, of the two resulting nuclei of this division, one 
lias the whole of the male part of the original nucleus and its 
share of the female portion, while the other has only half the 
female and no male substance. This appears to be the sex- 
determining factor in the sexual or fertilised egg*; for of 
these two daughter-nuclei, the one that has received the 
whole of the male element plus the female element becomes 
the nucleus of a female egg, while that which has received 
the egg portion becomes that of the male. Both these kinds 
of eggs, once the sex-determining division has taken place, 
grow rapidly (fig. 42). They seem to accomplish this through 
the power of absorbing all the other immature egg’-cells with 
which they happen to come in contact, and in which the divi- 
sions of the two portions of the male and female substance have 
been equal. The outcome of this process is that the male-pro- 
ducing egg is not fertilised while the female-producing egg is. 
It is, however, impossible to speak in the strict sense of the 
word of the male egg as unfertilised, as it has been directly 
under the influence of the sperm in all the early oogonial divi- 
sions previous to the sex-determining one. For all the primi- 
tive germ-cells are joined in the first place by a spermatozoon, 
irrespective of the fact that only some of these will give rise 
to ova later, and that the majority will be only nurse-cells. 
It is only in the late stages, shortly before the female egg is 
laid, that the two portions of the nucleus, the male and 
female, actually fuse beyond recognition. As the two kinds 
of eggs, male and female, are not found in the simple ratio, 
but in the proportion of three or two females to one male, it 
