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TEXT-BOOK OF EMBRYOLOGY. 



the nucleus. The chromatin granules become 

 concentrated into a dense mass in which very 

 little structure is made out (Fig. 10, A). After 

 the period of growth the nucleus assumes again 

 the reticular 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 com- 

 plete. 



The fused chromosomes now prepare for 

 division. However, instead of dividing longi- 

 tudinally into two parts, a double splitting 

 occurs and each chromosome is divided into 

 four elements. Such a quadruple chromosome 

 is termed a tetrad (Fig. 10, E, F, G). Since 

 each tetrad represents a double chromosome, 

 the number of tetrads in any species will be 

 equal to one-half its normal number of chro- 

 mosomes. The tetrads arrange themselves in 

 the equatorial plane of the spindle and cell 

 division begins (Fig. 10, G). Each tetrad is 

 separated into two dyads, and then one dyad 

 from each tetrad goes to each of the two re- 

 sulting daughter cells or secondary spermato- 

 cytes (Fig. 10, 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. Each dyad is 

 separated into two monads, each daughter 

 cell or spermatid receiving one monad from 



1-8, Spermatogonia lying close to the 



basement membrane and multiplying 



by ordinary mitosis. 9-16, Spermatogonia during period of growth, resulting in primary spermato- 

 cytes. 17, 18, 19, Primary spermatoscyte dividing. 20, Secondary spermatocytes. 21, Second- 

 ary spermatocytes dividing, resulting in spermatids (22-25). 26-31, Transformation of spermatids 

 into spermatozoa, a few of which are seen fully formed (32). 



