CYTOPLASMIC INCLUSIONS OF THE GERM-CELLS. 599 
phenomenon. It is the presence of two definite kinds of 
mitochondria, which apparently have a definite location in 
every sperm tail. This is a fact of cardinal importance, and 
seems to show that there is a division of labour or function; 
between the mitochondria of the spermatid. The position of 
the micromitochondria is more definite than that of any 
plasmatic structure other than centrosomes or perforatorium. 
Terni figures and describes his mitochondria throughout as 
rods, remarkably equal in size and length. He has observed 
fresh material, and therefore apparently this rod-like structure, 
which can be artificially produced in Helix by bad fixation, is 
the true state of affairs in Geotriton . 1 
c. The Determination of Oogonium, Spermato- 
gonium, or Nurse-Cell. 
In the following, the words “ determination of sex ” are 
used for the above for convenience. I have been unable to 
produce any definite evidence regarding the determination of 
sex. This paper deals almost exclusively with the plasma, 
but I have come to certain conclusions with regard to this 
important matter. In short, I am convinced that since it is 
the nucleus which is the first part of the indifferent cell to 
begin differentiation, we are justified in believing that the 
nucleus is the prime factor in the production of any one sex. 
The nucleus enlarges, its chromatin undergoes changes, a 
cloud appears at the edge of the nucleus, evidently under the 
influence of the latter, and every step in the metamorphosis 
of the cytoplasm is preceded by one in the nucleus. If we 
saw the nucleus remaining as it was for some time, waiting 
till the Nebenkern or mitochondria appeared, we would 
naturally think that the latter structures, after appearing, 
stimulated the nucleus. As it is the other way about, I feel 
justified in entertaining the view that it is not the plasma, 
1 Since this paper was written I have come to the conclusion that the 
rod-like mitochondria (PI. 33, fig. 34) where they occur are artefracts 
produced by bad fixation, but as I have not observed Triton I cannot 
venture to question Term's results. 
