TESTICLE-CELLS OF FUNDULUS 141 



between the daughter-nuclei (figs. 10 and 11). The same proc- 

 ess is repeated during the second division (fig. 12). Though the 

 cells are very small, it is easy enough to distinguish both mitoses 

 owing to the following characteristics. The first spermatocytes 

 are larger than the second ones. The spindle at the stage of 

 metaphase is more slender in the second division. The number 

 of chondriosomes decreases conspicuously. Finally the size and 

 shape of the chromosomes as observed in Benda's material pre- 

 sent a most distinctive character: in the first division they are 

 unmistakably heterotypic. 



The spermatids, which are exceedingly small, very soon form an 

 axile filament. At first the mitochondria are scattered all 

 around the nucleus but only for a short time. In the succeeding 

 stage which is very characteristic and which, judging from its 

 frequent occurrence in the preparations, lasts apparently a con- 

 siderable period, all the mitochondria are found accumulated in 

 one heap at the posterior pole of the nucleus where they surround 

 the proximal part of the axile filament (fig. 13). A glance at 

 these cells readily gives the impression that the number of their 

 mitochondria is constant. When one attempts to count them 

 however, one realizes that to obtain exact figures is almost im- 

 possible, for the granules are very small and not all in the same 

 level. The numbers I found in the most favorable cases came 

 very close to eight. 



Further stages of spermiogenesis are characterized by changes 

 in the mitochondria (which will be described below), the growth 

 of the tail and the following modifications of the nucleus. First, 

 the posterior side, which is in close contact with the mitochon- 

 dria, becomes flattened or even somewhat concave (fig. 14). 

 Its chromatic content then gradually accumulates at the periph- 

 ery, with the exception of the posterior or flattened side, a proc- 

 ess whose occurrence has been described several times in inver- 

 tebrates and which begins in Fundulus at the stage represented 

 by figure 14. The crust of chromatin thus formed assumes the 

 outline of a horse-shoe, the space existing between the free ends 

 of its branches being occupied by the mitochondria; from the 

 same space emerges the axile filament (fig. 15 et seq.). Later, 



