spermatogenesis 483 



elements present in the nucleus. Just before the contraction 

 stage, the spermatogonia! chromosomes were joined end to end by 

 linin connections, and out of the contraction stage there came a 

 continuous spireme, which has passed through various stages and 

 finally segmented. If the chromosomes conjugate end to end in 

 the late anaphase (Fig. 8), as Fig. 9 might suggest, the longitudinal 

 axis of the primary spermatocyte segments, or chromosomes, 

 represents the longitudinal axis of the spermatogonia! chromo- 

 somes. The presence of a massed anaphase and of the contrac- 

 tion stage makes it impossible to prove that this is the case here. 

 It has, however, been proved for other forms (Sutton) and the 

 agreement of all other steps in the process points to a possible 

 similarity in this respect also. The 10 segments next become 

 tetrads by the formation of transverse arms which always remain 

 a little shorter than the longitudinal arms, and thus make it always 

 possible to distinguish between the longitudinal and transverse 

 axes (Figs. 17 to 19 ). While the tetrads and dumb-bells are form- 

 ing, the odd chromosome rounds up again and becomes a lens- 

 shaped body, still applied to the nuclear membrane (Fig. 20). It 

 is in the dumb-bell form that the chromosomes usually enter the 

 spindle (Fig. 24), but occasionally they are still in the form of cross- 

 shaped tetrads (Fig. 22). This shows conclusively that the longi- 

 tudinal axis of the dumSbell is the same as the longitudinal axis of 

 the tetrad, and that the first spermatocyte mitosis is a transverse 

 division. That it is probably a reducing division can be shown by 

 tracing back the development, and working out the corresponding 

 axes: the division between the halves of the dumb-bell (Fig. 24) 

 corresponds to a division along the lateral arms of the tetrad (Fig. 

 17), and that to a transverse section of the spireme segment (Fig. 

 16) and that to the separation of one spermatogonia! chromosome 

 from another, if we assume that each spireme segment equals two 

 spermatogonial chromosomes joined end to end. This may be 

 further evidence against McClung's ('00) contention that the 

 reducing division is always the second. In the equatorial plate 

 of the first spermatocytes the odd chromosome stands a little apart 

 from the other 10 chromosomes, and is smaller in diameter (Fig. 

 21). It does not divide in the first spermatocyte division, but lags 



