274 GENETICS 



is the mule (figure 55). Of the two parents, the horse has 

 37 or 38 large chromosomes,'^ the ass 65 or 66 small ones. 

 These work well together in development : the mule is 

 perhaps more vigorous than cither parent. But when germ 

 cell formation begins in the mule, the two kinds of chromo- 

 somes at the time of conjugation partly or entirely destroy 



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Figure 55. Chromosomes of the mule in germ cell formation. A, 

 Early stage in germ cell formation, showing the large chromosomes 

 of the horse and the small chromosomes of the mule. 



B, Later stage of germ cell formation. Germ cell and chromosomes 

 degenerating; chromosomes irregularly scattered. After Wodsedalek 

 (1916). 



one another (figure 55, B). No functional germ cells are 

 formed. 



In other cases the sterility of the hybrids is not complete. 

 Some of the chromosomes of the two species refuse to 

 conjugate, while others do conjugate. The number that 

 conjugate successfully varies in different germ cells even of 

 the same hybrid individual. Some of the chromosomes are 

 injured or eliminated. Thus many of the germ cells formed 

 are imperfect; many die. The production of a large pro- 

 portion of imperfect germ cells is one of the commonest 

 features of hybrids. Some of the germ cells however may 

 live; these are presumably the ones in which the chromo- 

 somes have been least injured. Such germ cells may func- 



