2 MENDELIAN SEGREGATION 
carries a definite number of bodies called chromo- 
somes. The egg carries the same number. Conse- 
quently, when the sperm unites with the egg, the 
fertilized égg will contain the double number of 
chromosomes. For each chromosome contributed by 
the sperm there is a corresponding chromosome con- 
tributed by the egg, 7.e., there are two chromosomes 
of each kind, which constitute a pair (Fig. 1, a). 
When the fertilized egg divides, every chromo- 
some splits into two chromosomes, and these two 
daughter chromosomes then move apart, going to 
opposite poles of the dividing cell (Fig. 1, c). Thus 
each daughter cell (Fig. 1, d) receives one of the 
daughter chromosomes formed from each original 
chromosome. The same process occurs in all cell 
divisions, so that all the cells of the animal or plant 
come to contain the double set of chromosomes. 
The germ cells also have at first the double set of 
chromosomes, but when they are ready to go through 
the last stages of their transformation into sperm 
or eggs the chromosomes unite in pairs (Fig. 1, e). 
Then follows a different kind of division (Fig. 1, f) 
at which the chromosomes do not split but the © 
members of each pair of chromosomes separate and 
each member goes into one of the daughter cells 
(Fig. 1, 9, h). As a result each mature germ cell 
receives one or the other member of every pair of 
chromosomes and the number is reduced to half. 
Thus the behavior of the chromosomes parallels the 
behavior of the Mendelian units, for in the germ cells 
each unit derived from the father separates from the 
