232 CHARLES W. METZ 



long chromosomes, one shorter chromosome and one small, 

 spherical chromosome. Figures 53 to 57 illustrate correspond- 

 ing diploid groups of the same species taken from ovarian (53-56) 

 and somatic (57) cells. From these figures it is evident that 

 for each single chromosome of the haploid group there is a pair of 

 chromosomes in the diploid group, and that the members of this 

 pair are in close proximity to one another. Earlier stages, show- 

 ing the origin of the pairs in prophase, are given in figures 58-66. 

 Some of the figures represent only sections of the nucleus, but 

 others (58, 62, 63, 65, 66) are taken from uncut nuclei and in- 

 clude all of the chromatic material. In early prophase stages 

 the five pairs of long chromosomes are clearly represented by 

 the five long, double threads as shown in figures 58, 62 and 65. 

 Frequently the small pair is concealed and cannot be distin- 

 guished, but in niany cases it is as clearly evident as are the 

 others (figs. 62, 65). The duality of the threads in early pro- 

 phase is perfectly distinct in almost all cases. The figures given 

 here are entirely typical of scores studied, and are taken from 

 various tissues of the body all of which show the same phenomena 

 in dividing cells. Very rarely a figure is found in which no 

 duality can be seen in the threads (fig. 63), but it seems certain 

 that this appearance is due merely to overstaining which con- 

 ceals the true dual nature. Figure 65 is a good example of such 

 a case. When first studied the members of this group appeared 

 to be perfectly homogeneous elements and were drawn as such, 

 but after the material had cleared in balsam a few months, the 

 duality of the threads became very evident, as shown in the 

 figure. I have no hesitancy, therefore, in considering figure 

 63 to be of the same nature, especially since it is almost the only 

 clear case of its kind found. 



During later prophase stages such as shown in figures 53, 55 

 and 66, the chromosomes rapidly contract, and condense, and 

 the members of a pair dissociate somewhat in preparation for 

 division. When they go on the spindle they form a flat equatorial 

 plate, with corresponding chromosomes arranged side by side 

 in the same plane. Only in exceptional cases, such as are in- 



