43 2 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-division 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 



