412 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
however, the separation of the members of the chromosome pairs 
results in sperms half of which contain an X-chromosome and half a 
Y-chromosome in addition to the three autosomes. The reduction 
division in the male insures an equality in numbers for the two kinds 
of sperm cells and the chances that either kind of sperm will fertilize 
an egg-cell are equal. By this arrangement the numerical equality 
of the sexes is maintained. When, later, the egg cells of the female are 
fertilized by the sperm cells of the male, as shown in the lower portion 
of the figure, half of them being fertilized by sperm cells which contain 
an X-chromosome will give females, and half uniting with sperm cells 
which contain Y-chromosomes will produce males. The inheritance 
of sex in Drosophila provides a beautiful illustration of the parallel 
behavior of the chromosome mechanism and a somatic difference, in 
this case, sex. ; 
To recapitulate, the essential phenomena of cell behavior which fur- 
nish the mechanism for the distribution of hereditary factors are these: 
1. Every species is characterized by a definitely organized group 
of chromosomes. The chromosomes occur in pairs, in each of which 
one member is derived from each parent. In ordinary somatic mitosis 
the distribution of chromatin is such that each daughter cell receives 
a full complement of chromosomes which are equivalent qualitatively 
to those of the mother cell. 
2. In germ cell formation the homologous chromosomes conjugate 
during synapsis, then separate, and pass into a division figure in which 
entire homologous chromosomes are opposed to each other. The 
resulting reduction division gives daughter cells with half the number 
of chromosomes characteristic of the species, the half number being 
made up of one member of each pair of chromosomes. During synap- 
sis there is an opportunity for the members of a pair of chromosomes 
to exchange chromatin material. When such interchange takes place 
equivalent portions of chromosomes both qualitatively and quantita- 
tively are involved. In the reduction division segregation within one 
pair of chromosomes is entirely independent of that of any other pair 
so that the combinations of parental chromosomes in the germ cells 
represent all those to be expected on the basis of chance distribution. 
The student should constantly endeavor to harmonize this con- 
ception of the distributing mechanism of the chromatin material with 
the Mendelian interpretations of hereditary phemomena which will be 
presented in what follows, to the end that he may obtain a clear and 
definite idea of the interrelations between the known facts of heredity 
and cell behavior. 
