432 Evolution and Adaptation 
chromatin rods derived from the nucleus are divided crosswise 
(Fig. 6 B, C). The same thing occurs at one of the two 
divisions in the formation of the spermatozoon (Fig. 7 B, C). 
At the other division to form the other polar body (or the 
other sperm-cell) the chromatin rods appear to be split 
lengthwise, as in ordinary cell division (Fig. 6 E, F, G). 
In recent years the cross-diviston of the chromatin rods has 
attracted a great deal of notice, and Weismann in particular 
drew attention to the possible importance of this kind of 
division. 
There is another fact that gives this division especial sig- 
nificance. It has been discovered that the number of chro- 
mosomes that appears in each dividing cell of the organism 
is a constant number, but it has also been discovered that 
the egg, before extruding its polar bodies, and the mother- 
cell of the spermatozoon (Figs. 6, 7 B), contain exactly 
half of the number of chromosomes that are characteristic 
of the body-cells of the same animal (Figs. 6,7 A). Now 
there is good evidence to show that the reduction in number 
is due to the chromosomes uniting sometimes end to end in 
pairs, as shown in Figures A and B. Furthermore, it has 
been suggested that at one of the maturation divisions, when 
the chromosomes divide crosswise, the united chromosomes 
are separated (Figs. 6, 7 B, C), so that one remains in the 
egg and the other goes out into the polar body. The same 
thing is supposed to occur at one of the maturation divisions 
of the sperm mother-cell. A further consideration of capital 
importance in this connection has been advocated by Mont- 
gomery and by Sutton, namely, that, when the chromosomes 
unite in pairs, a chromosome from one parent unites with 
one from the other parent. Consequently at one of the two 
reduction divisions maternal and paternal chromosomes may 
separate again, some to go to one cell, some to the other. 
When the spermatozoon enters the egg it brings into the 
egg as many new chromosomes as the egg itself possesses at 
