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- 



