REPRODUCTION 561 



by the process of mitosis previously described (p. 24). At a cer- 

 tain stage, however, every germ-cell passes through a modified 

 mitosis to fit it for taking part in the reproductive cycle. An es- 

 sential feature of reproduction in higher forms, as noted in the 

 last paragraph, is fertilization, or the fusion of two germ-cells, 

 male and female. This fusion, by adding the chromosomes of the 

 male cell to those of the female, would double the number in the 

 fertilized egg were not some arrangement provided to avoid it. 

 This arrangement is found in the modified mitosis mentioned 

 above, to which is given the name maturation or ripening of the 

 germ-cell. We will recall from the earlier description of mitosis 

 that the chromatin forms itself into a definite number of 

 chromosomes, and that each chromosome splits lengthwise in 

 such fashion that half of it goes to each daughter cell. Thus the 

 daughter cells are exactly alike so far as chromosome content goes. 

 In the process of maturation the chromosomes do not split length- 

 wise. Instead half of them pass to one daughter cell and half to 

 the other. Thus we have a reduction of the number of chromo- 

 somes to half the original. Moreover the daughter cells of this 

 division are not alike in chromosome content, a fact that is sup- 

 posed to be highly significant in heredity, since the chromosomes 

 are looked upon as determiners of hereditary traits. 



In the formation of sperm the 'daughter cells of the maturation 

 or reduction division (so called because it reduces the number of 

 chromosomes), are of equal value. Each undergoes an additional 

 mitosis of the ordinary type, so that from each primary sperm- 

 forming cell four functional spermatozoa are derived. The ovum 

 is the active agent in the reproductive process. Its maturation 

 proceeds in such a fashion that the cell mass as a whole is undis- 

 turbed by the changes taking place in the chromatin. The reduc- 

 tion division occurs with the chromosomes of the ovum, precisely 

 as in the primary sperm-cell ; but instead of this chromosome divi- 

 sion being followed by ordinary cell division, the division is un- 

 equal. Most of the cell substance continues as before, retaining 

 half the chromosomes. A very small amount is set apart with 

 the other half of the chromosomes, and serves no useful purpose. 

 This is known as the first polar body. At this stage the ovum and 

 the polar body are comparable to the daughter cells of the reduc- 

 tion division of the primary sperm-forming cell. There is an addi- 



