250 



REDUCTION OF THE CHROMOSOMES 



the spireme-thread splits lengthwise before its segmentation into rods. 

 It then divides transversely to form six double rods (half the usual 

 number of chromosomes), which open out to form six closed rings. 

 These become small and thick, break each into four parts, and thus 



Fig. 123. Formation of the tetrads and polar bodies in Cyclops, slightly schematic. (The 

 full number of tetrads is not shown.) [RiJCKERT.] 



A. Germinal vesicle containing eight longitudinally split chromatin-rods (half the somatic 

 number). B. Shortening of the rods; transverse division (to form the tetrads) in progress. 

 C. Position of the tetrads in the first polar spindle, the longitudinal split horizontal. D. Ana- 

 phase ; longitudinal divisions of the tetrads. E. The first polar body formed; second polar 

 spindle with the eight dyads in position for the ensuing division, which will be a transverse or 

 reducing division. 



give rise to six typical tetrads. An essentially similar account of the 

 ring-formation is given by Vom Rath in Euchata and Calanus, and 

 by Riickert in Heterocope and Diaptomus. 



That the foregoing interpretation of the rings is correct, is beauti- 

 fully demonstrated by the observations of Hacker, and especially of 



