12 



TEXT-BOOK OF EMBRYOLOGY. 



6. Schematic outline of sper- 



into a dense mass in which very little struc- 

 ture is made out. After the period of 

 growth the nucleus assumes again the reticu- 

 lar appearance. Then when the spireme is 

 formed and segmentation occurs, previous 

 to division, only the haploid or one-half the 

 normal number of chromosomes appears. This 

 seems to be due to an actual fusion of 

 chromosomes by pairs, such fusion occurring 

 during the period of growth and being known 

 as synapsis of chromosomes. In some cases 

 the double nature of the chromosomes is still 

 visible while in other cases the fusion is 

 complete. 



The fused chromosomes now prepare for 

 division. However, instead of dividing 

 longitudinally into two parts, a double split- 

 ting occurs and each chromosome is divided 

 into four elements. Such a quadruple chro- 

 mosome is termed a tetrad. Since each tetrad 

 represents a double chromosome, the number 

 of tetrads in any species will be equal to one- 

 half its normal number of chromosomes (Fig. 

 7, D). The tetrads arrange themselves in the 

 equatorial plane of the spindle and cell division 

 begins (Fig. 7, E, F, G) . Each tetrad is sepa- 

 rated into two dyads, and then one dyad 

 from each tetrad goes to each of the two 

 resulting daughter cells or secondary sperma- 

 tocytes (Fig. 7, H)'. A new spindle is formed 

 in each of the secondary spermatocytes and 

 the cells divide again, without the return of 

 the nucleus to the resting stage. The dyads 

 go to the equatorial plane (Fig. 7, 7, /, K). 

 Each dyad is separated into two monads, 

 each daughter cell or spermatid receiving one 

 monad from each dyad (Fig. 7 ,L). Aprimary 



lying close to the basement 



membrane and multiplying by ordinary mitosis. 9-16, Spermatogonia during period 

 of growth, resulting in primary spermatocytes. 17, 18, 19, Primary spermatocytes divid- 

 ing. 20, Secondary spermatocytes. 21, Secondary spermatocytes dividing, resulting in 

 spermatids (22-25). 26-31, Transformation of spermatids into spermatozoa, a few of 

 which are seen fully formed (32). 



