THE PHYSICAL BASIS OF HEREDITY. 181 
of each tetrad. Since later stages show that the two 
chromatin masses in each spermatid of Fig. 11, /, repre- 
sents two chromosomes, we see that the number of chro- 
mosomes has been reduced from the four in Fig. 11, 4, to 
two in Fig. 11, /. Manifestly the key to the explanation 
lies in the relations which exist between the four chro- 
mosomes of Fig. 11, 4, and the tetrads of Fig. 11, 2. 
The two divisions consist merely in the distribution of 
the already separated parts of the tetrads; in the vear- 
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 ts 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 
taking place in the rearrangement of 
the chromosomes into the tetrads. To 
Riickert we owe the clearest account of the process as 
exhibited in the egg maturation of Cyc/ofs. Here the 
normal number is 22, or perhaps 24, the minute size 
rendering counting difficult. In Fig. 12, A to F, taken 
from Riickert, 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, B, 
which in Fig. 12, C, are being arranged in the developing 
spindle. A comparison of these three figures will show 
Reduction in 
Crustacea. 
