ESSENTIAL FEATURES OF THE SEXUAL PROCESS 1259 



characteristic of the species, and since these chromosomes would divide by 

 splitting, the number of chromosomes in each cell would be doubled with 

 each generation. This doubling is obviated by the fact that, in the forma- 

 tion of the germ cells, the ovum and spermatozoon nuclei undergo a special 

 type of division, which leads to the reduction of the chromosomes in the 

 sexually mature cell to one-half of the number characteristic of the species. 

 This mode of cell division is often called ' division by reduction,' or 

 ' heterotype ' mitosis, or ' meiosis ' (Fig. 569). 



We may take as an example the development of spermatozoa. The mother cells 

 of the spermatozoa, the spermatocytes, divide twice, giving rise to four daughter cells, 

 the spermatids, each of which develops into a functional spermatozoon. In the nuclear 



FIG. 569. Three stages of heterotype mitosis in spermatocyte of triton. (MooRE.) 



a, germinal condition of chromosomes; b, gemini arranged in quadrate loops or 

 tetrads; c, separation of tetrads into the duplex chromosomes of the daughter 

 nuclei. 



changes preparatory to the first division, the spireme, when it breaks up, gives rise 

 to only half the normal number of chromosomes. Thus if the somatic number of chro- 

 mosomes were four, we should find in the spermatocyte, after the breaking up of the 

 spireme, only two chromosomes. These take up their position at the equator of the 

 achromatic spindle and then divide ; the division is effected however, not by splitting of 

 the double chromosome, but by transverse division. Each chromosome breaks into half, 

 one half going to each daughter cell. Since each of the reduced number of chromosomes 

 can be regarded as made up of two normal chromosomes placed end to end or joined 

 to form a ring, as in Fig. 569, b, the division in the middle provides for a qualitative 

 difference between the two daughter cells. If we indicate the four normal chromosomes 

 as a, b, c, d,m ordinary somatic division, each daughter cell will also contain chromosomes 

 which may be represented as ol, 61, cl, dl, and a2, 62, c2, d2. In the spermatocyte 

 the two chromosomes may be represented as db and cd. When they divide, one daughter 

 cell receives a and c, while the other daughter cell receives b and d. The second division 

 of these daughter cells takes place generally by splitting of the filaments, so that finally 

 four spermatids are produced (Fig. 570), each with two chromosomes, two of them 

 containing a and c, while the other two contain b and d. In the ovum, during matura- 

 tion, analogous changes take place. Two successive cell divisions occur as in the forma- 

 tion of spermatozoa, but the daughter cells are of very unequal size. In the first division, 



