SEX DETERMINATION IN DINOPHILUS GYROCILIATUS. 357 
divided and drawn apart. In fig. 17 a later step in the same 
process is shown from another section. Here the male 
portion (m. 'pro.) has already divided and moved apart, while 
the female portion has drawn out and is about to divide, as 
shown in the text-figure (fig. 18). Finally, in fig. 19 the two 
separate nuclei are shown. The darkly staining male portion 
is clearly distinguishable, while the female portion is repre- 
sented by a number of chromatic granules distributed 
throughout the nucleus and not very clearly shown in the 
figure. It is thus seen that in all these nuclear divisions the 
male and female portions of the nucleus divide separately. 
In the majority the division of the male substance precedes 
that of the female. On referring to fig. 22, it will be seen 
that each female portion of the nucleus has the small portion 
beside it, within the same nuclear well. In most instances 
the male portion has already divided and is represented by 
two black masses which are separating. Similarly, in fig. 21, 
the male and female part of each nucleus can be distinguished. 
In fig. 28 are shown two nuclei which have just divided and 
separated, in a small mass of cytoplasm ; here the structure 
of the nucleus is well shown. Fig. 29 represents a similar 
nucleus under greater magnification, in which the separation 
of the male and female portions is again shown. Fig. 35 
represents still another series of nuclei ; while in fig. 34 an 
early stage in the division of the male portion of the nucleus 
is shown. 
4. Late Oogenesis and Formation of Male and Female Eggs. 
As a rule the nuclei in such a mass as that shown in fig. 22 
divide equally ; that is, an equal portion of male and female 
chromatic substance goes into each daughter-nucleus. As 
long as these nuclear divisions continue to be equal no 
change takes place beyond that of slow growth. But now 
and again the divisions can be seen to be distinctly unequal, 
that is, for some reason or other the female portion has 
divided before the male, and this last has gone wholly over, 
