204 



EVOLUTION, GENETICS, AND EUGENICS 



mechanism of mitosis being used. After many thousands of germ 

 cells have been produced, multiplication is stopped and growth sets in, 

 the growth in the male cells being very slight as compared with that of 

 female cells. During the period of growth the chromosomes not only 

 split to form twin chromosomes, as though in preparation for mito- 

 sis, but whole homologous chromosomes unite in pairs, a process known 



o'6 g 



Fig. 46. — The germ track in Miastor americana. A, germ-cell (p-g.c.) set apart 

 in the eight-celled stage of cleavage. (After Hegner.) The walls of the remaining 

 seven somatic cells have not yet formed, though the resting or the dividing (M p) 

 nuclei may be seen; C R, chromatin fragments cast off from the somatic cells; B, 

 section lengthwise of a later embryo of Miastor; the primordial egg-cells (obg 3 ) are 

 conspicuous. (From Guyer, after Hegner.) 



as synapsis. The result is that, instead of the set of single chromosomes 

 characteristic of the zygote, there are groups of four (tetrads), each 

 tetrad composed of two pairs of twin chromosomes bound together. 

 There are just half as many tetrads as there are chromosomes in the 

 zygote. Now follow two special cell divisions, known as meiosis, with- 

 out any further change in the chromosomes. The result is that one 

 chromosome of each tetrad goes to each of the four matured sperm 

 cells. In one of these divisions, the so-called reduction division, both 

 twins of an originally single chromosome pass together to one cell, 



