THE BRIDGE OF LIFE 493 



traction between homologous chromosomes so that they come together 

 and form 24 pairs. Each chromosome becomes duplicated as in ordi- 

 nary mitosis, however, so each of the 24 pairs consists of four chro- 

 matids, but with only two centromeres. These tetrads line up at the 

 metaphase, but there is no duplication of centromeres as in ordinary 

 mitosis. The paired chromosomes are then pulled apart in the anaphase, 

 and each daughter cell receives 24 chromosomes. 



Meiosis is not yet complete, however, for each of these is a double 

 chromosome (dyad) with two chromatids. Another prophase follows 

 without any further duplication of chromatids, and the chromosomes 

 line up at the metaphase spindle with two chromatids and one centro- 

 mere for each chromosome as in ordinary mitosis. At this metaphase 

 the centromeres become duplicated and the chromatids are pulled apart 

 to form 24 single chromosomes for each daughter cell that is formed. 

 Thus, in two cell divisions the number of chromosomes and the num- 

 ber of genes are reduced to one half. Gametes are then formed from 

 these cells. 



Spermatogenesis 



This is the term used to refer to sperm production. Since there is 

 some difference in the way in which meiosis occurs in the male and 

 female, we will consider the two separately. A cell of the testes which 

 is preparing to undergo meiosis is known as a primary spermatocyte. 

 The first division of meiosis results in two secondary spermatocytes 

 of equal size. These both undergo the second division of meiosis and 

 produce four spermatids. Each of these is then converted into a 

 sperm by an elimination of most of the cytoplasm, the concentration of 

 the genes into a head and the development of a tail for locomotion. 



Every male animal produces enormous numbers of sperms. Any 

 one sperm has little chance of fertilizing an egg, and there must be 

 enough of them to insure their presence at the proper place at the proper 

 time. There will be enough sperms released by a man at a single 

 ejaculation to fertilize every woman on this hemisphere of the earth 

 if every single one were used. Thus, it is evident that the sperms are 

 quite small, but they contain all that is needed. They contain the im- 

 portant genes concentrated in the head, a little stored food in a neck, 

 and a long whip-like tail for locomotion. In spite of their small size, 

 however, we should bear in mind that they contain just as many genes 

 as there are in the female egg which is hundreds of times larger. Thus, 

 each child inherits equally from father and mother, because the genes 

 are the units of heredity. 



