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CONTINUITY OF LIFE 



Fig. 22-4. This is a section of the mammalian testis (rat), showing a seminiferous tubule in detail. Sperm cells begin 

 their development at the outer edges of the tubule and as they mature they move into the cavity. The dark rod- 

 shaped bodies nearest the cavity are mature sperms. 



diploid with respect to chromosome num- 

 ber. They undergo successive mitoses to 

 build up their population and move toward 

 the lumen (cavity) of the tubule to be- 

 come mature spenns. It is during this 

 period that their number of chromosomes 

 is reduced to the haploid condition. By fol- 

 lowing these cells it is possible to determine 

 how this reduction occurs. This is done 

 schematically in Fig. 22-5. 



Because a great many sperms are pro- 

 duced, the spermatogonia undergo many 

 divisions before launching into the final two 

 divisions that bring about maturation and 

 the production of mature sperm. At the 

 end of these preliminary division stages, 

 the spermatogonia increase in size and the 

 homologous chromosomes pair up in a proc- 



ess called synapsis. The pairing occurs only 

 between like (homologous) chromosomes, 

 one from the paternal parent and one from 

 the maternal parent. Then each clnomo- 

 some, while in this paired condition, dupli- 

 cates itself so there are as a result four 

 united chromosomes. Duplication may have 

 occurred at some earlier period but it is 

 visible only at this time. These are called 

 tetrads, and the cells containing them are 

 called primary spermatocytes. These cells 

 then divide, carrying paired chromosomes, 

 now called dyads, to each daughter cell, or 

 secondary spermatocyte. A second division 

 occurs immediately in which the paired 

 dyads separate into monads, so that each 

 daughter cell, now known as a spermatid, 

 contains one set of chromosomes only in- 



