THE PHYSICAL BASIS OF MENDELISM 67 
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 differ- 
ence, 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 definite number of chromosomes, 
each of which is made up of a definitely organized group of chromomeres. 
The chromosomes occur in pairs, in each of which one member is derived 
from each parent. In ordinary somatic mitosis the distribution of chro- 
matin is such that each daughter cell receives a full complement of chro- 
mosomes 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 re- 
sulting 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 synapsis there 
is an opportunity for the members of a pair of chromosomes to ex- 
change chromatin material. When such interchange takes place equiva- 
lent portions of chromosomes both qualitatively and quantitatively are 
involved. In the reduction division segregation within one pair of chro- 
mosomes 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 conception 
of the distributing mechanism of the chromatin material with the Men- 
delian interpretations of hereditary phenomena 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. 
Digitized by Microsoft® 
