Heredity and its Bearings on Atavism. 135 
mother-nucleus. The two mother-cells, by a similar division, 
give rise to four daughter-cells. « 
Each daughter-cell, however, which is going to become 
the mother-cell of two sexual cells, does not undergo 
division in the usual manner, 2.¢., its nuclear segments do not 
undergo a preliminary division, but are simply distributed 
between the two sex cells, each of which will therefore 
receive only two segments. This reduction in the number 
of chromatin segments is characteristic of sex cells, and as, 
further, the reduction is always equal to half the number of 
segments found during the non-sexual condition, it is justi- 
fiable to call the nuclei of sex cells, half or demi-nuclei. 
A grandmother-cell will thus always give rise to eight sex 
cells, but the fate of these latter will depend on whether 
they are to become male or female cells. If they are to be 
male cells, all the eight will, as a rule, attain maturity, while, 
if the grandmother-cell give rise to an egg, only one cell 
is fully developed, the ovum proper, while the three others 
are non-functional, and are termed the polar vesicles or 
Liichtungskorper. 
That only one-half the number of the chromatin segments 
found during the non-sexual condition is handed on from both 
parents is a well-established fact; and I only wish that as 
sound an explanation could be offered, but as yet we are not 
able to do so. All teleological considerations, such as those 
offered by Weismann, have, of course, to be put aside. A 
sperm-nucleus or an egg-nucleus does not get rid of one-half 
the number of its segments to prevent overcrowding of 
ancestral qualities or predispositions, and at the same time 
to make room for the nuclear segments of the other sex. We 
must rather look out for some phase in the life-history of 
lower unicellular organisms in which a reduction of nuclear 
segments is found quite apart from any sexual act, ey., 
perhaps during the zoospore stage, for I believe that fecunda- 
tion is only a secondary adaptation between cells, the organs 
of which have become reduced in number or efficiency by 
some intrinsic or extrinsic factor which is detrimental to the 
normal existence of cell-life. As such a factor we might 
consider any cause leading to diminished metabolic changes 
