THE ODD CHROMOSOME IN CERASTIPSOCUS VI -.\<^US. 12J 



tion. In prophase there are 8 bivalent chromosomes, appearing 

 as dumbbells or tetrads while one odd chromosome renmins 

 round or slightly oblong with no signs of approaching division 

 (Figs. 6 and 7). In metaphase as tin- dumbbells and tetrads 

 become grouped at the equator of the spindle, the odd chromo- 

 some often still lags to one side of the equator (Figs. 8 and 9). 

 In all these prophase and metaphase groups, the chromosomes 

 are so large and distinct that there is no difficulty in being sure 

 of the full number, 8 dumbbells or tetrads and one odd chromo- 

 some w T ith no sign of division. The dumbbells are probably side 

 views of the tetrads, as Fig. 8 shows nearly all tetrads. In the 

 equatorial plates, the odd chromosome appears off to one side 

 (Fig. 10). In anaphase, it sometimes precedes (Fig. n), and 

 sometimes follows (Fig. 12) the other chromosomes to one pole 

 of the spindle. Fig. 13 shows the telophase of this division : the 

 odd chromosome stands at one end of the spindle in one secondary 

 spermatocyte, with no mate in the corresponding secondary 

 spermatocyte. In Fig. 14, the chromosomes can be counted in 

 the two reconstructing nuclei of the two cell products of one 

 primary spermatocyte division: 8 dumbbells on one side, 8 

 dumbbells and one single odd chromosome on the other. Each 

 dumbbell in the secondary spermatocyte represents half of a 

 tetrad. Thus the plane of the second spermatocyte division 

 was indicated already in the prophase of the first spermatocyte 

 division, and still is in the prophase of the second, but there is no 

 way to tell which is the reducing division. Among these telo- 

 phase stages in one individual were a few abnormally large cells 

 in which the mitotic mechanism had somewhere failed, and there 

 was an unreduced number of chromosomes (Fig. 15). Here 

 there are 16 dumbbells and the odd chromosome is recognizable 

 from its round shape and its position to one side of the group. 

 By the time the odd chromosome gets into the equatorial plate 

 for the second spermatocyte division it is also a dumbbell in 

 shape and cannot be distinguished from the others. Nothing 

 but dumbbells appear on the secondary spermatocyte spindle, 

 so the odd chromosome must divide in this division (Fig. 16). 

 The equatorial plates of the secondary >permatocytes show the 

 expected difference in the number of chromosomes, some & 



