DAVIS: SPERMATOGENESIS. 97 



cases it is usually possible to recognize three autosomes of the rod type, 

 two of which are much smaller than the other chromosomes, while 

 the third is medium sized; three autosomes of the cross tyjDe, of 

 which two are medium sized and the third is one of the larger chromo- 

 somes ; and five autosomes of the ring and crossed-loop type, of which 

 four are large and one medium sized. 



As the chromosomes divide, the rod-shaped forms first become 

 dumbbell-shaped by the formation of a transverse constriction, which 

 continues to deepen until the two components are entirely separated. 

 The cross-shaped chromosomes first become converted into rods by 

 an elongation in the direction of the spindle axis and a corresponding 

 shortening of the transverse arms. Later thev become dumbbell- 

 shaped and finally divide as in the former case. In the rings and 

 crossed loops the process is more complicated. The ends of the univ- 

 alent components are attached to mantle fibers connected with the 

 more distant pole. Consequently as the chromosome divides its 

 univalent components are pulled past each other (Plate 4, Figs. 63, 

 65, Plate 7, Figs. 179, 180). In all cases as the univalent components 

 separate the longitudinal split, which has been temporarily hidden, 

 again appears so that the chromosomes as they move toward the pole 

 are composed of two rods, which may lie parallel or may diverge at 

 the ends nearest the equator of the spindle, giving rise to the well known 

 V-shaped chromosomes (Fig. 65, Fig. M) so characteristic of the het- 

 erotypical mitosis. As the ring- or loop-shaped autosomes (Figs. 181, 

 182) are pulled out during division and the longitudinal spht reap- 

 pears, it can be plainly seen in favorable cases that the rods resulting-^ 

 from the longitudinal split do not lie parallel but are crossed at the 

 middle of the dividing autosome. Later the autosomes divide at this 

 point, which, as the slight enlargement indicates, is the place where 

 the univalent components are joined. I regard this crossed condition 

 of the two longitudinal halves of the autosomes as strong confirmatory 

 eAidence of my interpretation of their structure. 



If the reader will take two flexible, parallel wires, or stiff cords, and 

 bend them until the ends are crossed (Fig. Oa), thus forming a loop, 

 as in the case of the tetrads, he can get a much clearer idea of the 

 method of separation than by any description. Now, by pulling the 

 two ends in opposite directions the loop will diminish in size until a 

 kink (Fig, Ob) develops in the middle, and when the wires are fully 

 straightened out (Fig. Oc) they will be crossed just as the autosomes 

 are in Figures 181 and 182. 



As a result of their arrangement on the spindle, the univalent com- 



