SEX DETERMINATION IN PINOPHILUS GYROCILIATUS. 365 
of the oogonial cells is of a double nature, one portion of 
which is derived from the sperm, while the other is derived 
from the egg. 
(3) In the nuclear divisions that follow the growth and 
fusion of the oogonial cells the two portions of the nucleus 
divide equally. 
(4) In some of these divisions the female portion of the 
nucleus divides before the male and passes over wholly 
to one of the nuclei; thus one of the nuclei comes to 
possess half the female portion of the original chromatic 
substance together with the whole of the male portion, whilst 
the other, has only half the original female portion. This 
appears to be the sex-determining division. All the other 
nuclei of the oogonial mass in which this division has taken 
place then degenerate. The one which possesses only the 
female portion of the chromatic substance gives rise to the 
male, whilst the other, possessing the whole of the male in 
addition to the female portion, gives rise to the female. 
(5) In the early male and female egg s, the dimorphism of 
which is already marked, these two portions of the nucleus 
still remain distinct. The male egg possesses only a single 
nucleus, while the female has a double one. 
(6) During the late development of the female egg these 
two portions of the nucleus fuse beyond recognition. 
(7) Maturation takes place after the fusion of the male 
and female chromatic substance in the female egg and not 
before. 
(8) There are twenty somatic chromosomes in the male 
and female. 
(9) Two polar bodies are given off by both the male and 
the female egg. The first in each case dividing after being 
given off. 
(10) In the female egg twenty chromosomes go out in 
the first polar body and twenty remain in the egg ; actually, 
ten double chromosomes go out and ten double chromosomes 
remain in the egg. In the male egg the same thing takes 
place. In the second polar body, in the case of the male egg 
