150 EZRA ALLEN 



the other two tetrads being crosses, especially in view of the 

 fact that not more than two crosses have been seen in any one 

 cell. However, this statement is not to be interpreted as exclud- 

 ing the possibility of the occurrence of more rings and fewer 

 crosses, or vice versa, in any one cell. 



4. The sizes of the first spermatocyte chromosomes. When 

 classed according to size, these chromosomes form three groups, 

 more or less well differentiated: 5 small, 7 intermediate, and 7 

 large. These may be noted in the polar views, figures 18, 19, 

 and 20; also in figure 27. The seven large ones appear readily 

 distinguishable in figures 19 and 20. In figure 18 the lines be- 

 tween the three groups are very difficult to draw. In figure 27 

 the seven large ones may be distinguished. But the line be- 

 tween the intermediate and small is always difficult to establish. 



While the accessory would appear in the equatorial plate 

 views to belong to the large group, one needs to remember its 

 less dense condition and its curved-rod shape, the curve of which 

 is not as strong as that of the rings. Moreover, the ring chromo- 

 somes are extended in two directions by the lUgs, the lengths of 

 which do not directly appear in the polar views. 



This grouping according to size corresponds with that of the 

 spermatogonial chromosomes (page 137). These divided them- 

 selves into three groups: 10 small, 13 intermediate, and 14 long. 

 Likewise we meet again the same difficulty in drawing the line 

 between the small and intermediate groups. 



5. Arrangement on the spindle. The most conspicuous chromo- 

 some on the spindle in either polar or lateral views, is often the 

 accessory, as it projects from the edge. This characteristic 

 position is shown in figures 18, 19, 20, 54 to 56, and 58. These 

 drawings and photographs are taken from four different rats. 

 The position was the same in the other animals examined. A 

 small ring chromosome appears close beside the accessory. One 

 of the large chromosomes, but not the largest, usually lies near 

 the accessory, as figures 18, 19, 20, and 55 indicate. 



The large ring chromosomes and the accessory group them- 

 selves about the periphery. The crosses lie similarly at times, 

 but they are not to be distinguished readily in polar views and 



