74 Experimental Zoology 
incapable of developing, and only the egg continues the history 
of the race. Half of the eggs, however, will on the average con- 
tain a particular kind of chromosome, and the other half the 
homologous kind, as shown in Figs II and II A. 
It is of special importance to notice in this connection that the 
pairs of chromosomes are assumed to lie haphazard on the spindle, 
so that while in one pair the maternal chromosome may be turned 
toward a given pole, in another the paternal chromosome may 
be turned toward the same pole. In other words, there are 
no grounds for assuming that all the paternal chromosomes 
turn toward one pole, and all the maternal toward the other, 
but ‘‘accident” alone determines which way they come to lie 
on the spindle. Hence the possibility of various combinations 
of chromosomes in the different cells is given. The evidence 
in the favor of the assumption of the accidental position of the 
chromosomes is indirect, and is deduced from the way in which 
the characters appear in the offspring of Mendelian hybrids 
when more than a single character is taken into account. 
Without this assumption the chromosomal hypothesis given 
above will not apply for more than one character. Whether 
this assumption is entirely satisfactory, will be considered later. 
A special case may make this discussion clearer. Let us 
assume that the character albinism of a white mouse is contained 
in one chromosome, and the gray character of the gray mouse 
in the homologous chromosome of the gray mouse. When 
these individuals are bred together the white chromosome, so 
to speak, and the gray chromosome are both present in the 
fertilized egg, which gives rise to the gray hybrid of the first 
generation, because the gray dominates the white. In the germ- 
cells of these gray hybrids the changes described above take 
place. At the synapsis stage the white chromosome pairs with 
the gray chromosome. Later, at one of the maturation divi- 
sions, the two separate and go to separate cells. Hence each 
germ-cell becomes “pure” and carries only one kind of 
color. If these hybrids (F,) with white and with gray germ-cells 
are paired, there will be formed by chance unions of the germ- 
