THE PHYSICAL BASIS OF HEREDITY. iSl 



of each tetrad. Since later stages show that the two 

 chromatin masses in each spermatid of Fig. n, F, repre- 

 sents two chromosomes, we see that the number of chro- 

 mosomes has been reduced from the four in Fig. n, A, to 

 two in Fig. 1 1, F. Manifestly the key to the explanation 

 lies in the relations which exist between the four chro- 

 mosomes of Fig. n, A, and the tetrads of Fig. n, B. 

 The two divisions consist merely in the distribution of 

 the already separated parts of the tetrads ; in the rear- 

 rangement of the four chromosomes into the two tetrads 

 lies the possibility of the reduction which is carried out 

 by the following divisions. The problem thus resolves 

 itself into the question, What is the nature of each tetrad? 

 Is it made up of a single chromosome, of two, of four, 

 or have the constituent parts of the original four chro- 

 mosomes become so completely rearranged and redis- 

 tributed that their identity as such is completely lost ? 



Turning for a moment to the lower Crustacea, we find 

 among the Copepods forms admirably suited for the 



careful following out of the changes 

 Reduction in taking p i ace i n the rearrangement of 

 Crustacea. 



the chromosomes into the tetrads. To 



Ruckert we owe the clearest account of the process as 

 exhibited in the egg maturation of Cyclops. Here the 

 normal number is 22, or perhaps 24, the minute size 

 rendering counting difficult. In Fig. 12, A to F, taken 

 from Ruckert, give the essential points of the forma- 

 tion of the tetrads and their following divisions, not 

 all the chromosomes being represented. In Fig. 12, A, 

 the chromatin filament has broken up into one half the 

 usual number of segments (chromosomes), and each 

 shows the precocious longitudinal splitting. These 

 segments shorten up into the double rods of Fig. 12, , 

 which in Fig. 12, C, are being arranged in the developing 

 spindle. A comparison of these three figures will show 



