SUTTON: SPERMATOGONIAL DIVISIONS IN BRACHYSTOLA MAGNA. 155 



collecting duct ; next comes the zone of the spermatids, then the sper- 

 matocytes, and then, when they are present at all, the secondary and 

 primary spermatogonia. 



2. Ohier cysts are always nearer the ?v^v (h'fercus than those of later 

 develoiDment. except in the zone of the spermatogonia, the center of 

 which is occupied by primary spermatogonia, while the cysts of sec- 

 ondary spermatogonia are arranged irregularly between the latter and 

 the follicular wall. 



o. Primary spermatogonia divide by mitosis. They may be dis- 

 tinguished from the secondary variety by their smooth, oval, vesicular 

 nuclei, and their relatively small amount of chromatin, as well as by 

 the fact that they are disposed irregularly and not in cysts, as are the 

 older generations. Most of the primary spermatogonia ultimately, 

 produce spermatozoa, but a few remain in the resting stage and, mov- 

 ing with the mature cj'sts toward the vas deferens, supply these mem- 

 branes with nourishment. 



4. In prei)aration for the last division, the flakes of chromatin of 

 the resting primary spermatogonia! nucleus become diffused and a 

 number of spiremes are formed. These become shorter, split longi- 

 tudinally, and then assume the form of short split or double rods — 

 apparently all alike. There is no nucleolus. The nuclear membrane 

 now dissolves, the chromosomes arrange themselves in the equatorial 

 plate, and the spindle, wiih its two tiny, deeply-staining centrosoraes, 

 may be clearly seen. The chromosomes next divide along the line 

 marked out in the prophase, and the halves are drawn to their respect- 

 ive poles, thus producing two secondary spermatogonia. Around 

 these two cells a membrane appears, forming a two-cell spermatocyst. 



5. Each chromosome, on reaching the pole, begins to disintegrate, 

 and, at the same time, reconstructs its share of the nuclear membrane 

 as a closed vesicle about itself. Later, all the vesicles become inter- 

 communicating at their polar extremities, with the exception of one, 

 which remains absolutely independent throughout its entire existence. 

 The chromatin of the ordinary chromosomes becomes diffused evenly 

 in the nuclear space, while that of the one in the separate vesicle (the 

 accessory chromosome) is deposited upon the inner surface of its 

 capsule. 



6. Following this, the chromatin of the joined vesicles goes through 

 the fine and coarse spireme stages, forming in each of the sacculations 

 of the nucleus a thick, spiral segment which splits longitudinally, and 

 afterward becomes a chromosome like that of the primary spermato- 

 gonia. The accessory chromosome has no spireme stage, but merely 

 collects in the form of a straight or twisted rod, which splits after the 

 manner of the others. All the chromosomes line up in the same man- 



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