1917J Tocom: Some Phases of Spermatogenesis in the Mouse 377 



In polar vews of the primary sperinatocyte spiudle, one chroiiiosoiiie 

 appears larger than the others ; and in side views of similar spindles, 

 one chromosome does not have the distinct (|uadripartite form, but 

 is more rounded. In the first division tliere is no indication that any 

 chromosome does not divide and but a slight indication of any pre- 

 cocious splitting. Figure 3 (pl. 30) shows one chromosome marked x 

 which does begin its migration slightly ahead of the others, but not 

 to a very marked extent. In the second division there is a chromosome 

 with a somewhat different behavior. Without division it begins its 

 migration ahead of the others and may have advanced half of the 

 way to the pole before the allosonies begin tlieir sepai'ation. This is 

 different from the behavior of the accessoi'v chromosomes, as described 

 by Wodsedalek (1913) in the pig and hor.se, and by Jordan (1911) 

 in the opossum. In these forms the accessory chromosome divides, not 

 in the primary spermatocyte, but in the secondary. This would make 

 the reduction division in the first spermatocyte. Secondary spermato- 

 cytes are therefore dimorphic. In the mouse, dimorphism is not 

 brought about until the formation of the spermatids, half of which 

 have nineteen chromosomes and half twenty. 



SPINDLE FIBRES AND CENTROSOME 



In sections stained with phosphotungstic-acid haematoxylin, the 

 spindle fibres show with remarkable clearness, while with the alcoholic 

 haematin stain they can be seen only faintly. In the second division 

 the Zwischenkorper is very conspicuous (fig. 9), but in the earlier 

 divisions it has not been noticed. 



A structure which is ever interesting is tlie centrosonie. When 

 stained by the phosphotungstic-acid haematoxylin, this structure ap- 

 pears as a small, bright red granule, toward which the spindle fibres 

 converge. The granule seems to be single in the primary spermato- 

 cytes, but in the secondary cells there is some indication that it divides 

 before the division of the chromosomes (pl. 30, fig. 8). If this is true, 

 it enters the spermatid as a double granule. 



