THE PHYSICAL BASIS OF MEN DELI SM 67 



the members of the chromosome pairs results in sperms half of which 

 contain an .X -chromosome and half a ^-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 JT-chromosome will give females, and half uniting 

 with sperm cells which contain F-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- 

 rnatin 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. 



