134 OUTLINES OF EVOLUTIONARY BIOLOGY 



FIG. 66. Some Stages in the Spermatogeuesici 01 a urasshopper (Stenolothrus 

 viridulus). (After Meek.) 



1. A secondary spermatogonium in mitosis, with seventeen chromosomes (eight pairs and 



one accessory). Polar view. 



2. Resting or growth stage following the mitosis of the secondary spermatogonium. The 



ordinary chromosomes have become resolved into a network but the accessory 

 chromosome retains its individuality. 



3. Primary spermatocyte formed by growth of 2, preparing for mitosis. 



4. Mitosis of the primary spermatocyte, showing the pairing (synapsis) of the sixteen 



ordinary chromosomes on the nuclear spindle and the accessory chromosome 

 unpaired. Side view. 



6. Later stage in the mitosis of the primary spermatocyte, showing separation of the 

 paired chromosomes. Side view. 



6. Still later stage in the division of the primary spermatocyte into two secondary sper- 



matocytes ; the chromosomes massing at the two poles of the spindle (one mass only 

 will contain the accessory chromosome). Side view. 



7. A secondary spermatocyte in mitosis, showing the reduced number of ordinary chromo- 



somes (S) and the accessory chromosome. (The sister-cell would of course contain no ' 

 accessory chromosome). Polar view. 



8. Later stage in the mitosis of the secondary spermatocyte ; each chromosome (including 



the accessory one, which lags behind the others) has split into two parts and the two 

 groups are separating. Side view. 



9. A spermatid, formed by division of a secondary spermatocyte with an accessory 



chromosome. It will form a single spermatozoon. 



x The accessory chromosome. 



I 



