!4 J. E. WODSEDALEK. 



The number of chromosomes in the spermatogonial metaphase 

 stage of the mule is, as was stated before, fifty-one, or fifty 

 autosomes and one accessory. The number in the corresponding 

 stage of the horse is thirty-seven or thirty-six autosomes and 

 one accessory; while the reduced number is eighteen in the one 

 type of sperm and nineteen, including the accessory, in the other. 

 According to the previous discussion it seems fairly safe to predict 

 that in the oogonial cells of the mare there are thirty-eight chro- 

 mosomes or thirty-six ordinary chromosomes and two accessories, 

 and that the reduced number in the matura ova is nineteen in- 

 cluding the one accessory. This would suggest that nineteen 

 of the fifty-one chromosomes including the accessory are maternal 

 in origin or contributed by the dam, and the remainder or thirty- 

 two are paternal in origin or contributed by the jack. On the 

 customary inference that the reduced number of chromosomes is 

 always one half the number in the spermatogonial and somatic 

 cells we should expect to find sixty-four chromosomes in the ass, 

 with the possible addition of one accessory in the jack and two in 

 the jennet; thus making a total of sixty-five and sixty-six re- 

 spectively in the two sexes of this animal. 



These plausible figures for the jack and jennet would, of 

 course, be expected only in the event that no cellular abnormalities 

 occurred in the mule from the time of the fertilization of the 

 dam's ovum by the jack's sperm, through the succeeding de- 

 velopmental stages of the hybrid up to maturity. In such a case 

 the full quota of chromosomes would be handed on to the sper- 

 matogonial cells of the mule. 



The normal mitotic figures of the spermatogonial cells seem 

 to indicate that probably no such abnormalities occur in the 

 various preceding stages. But even such an apparently normal 

 condition cannot be relied upon too strongly. For while the 

 spermatogonial cells are normal during mitosis, as near as can be 

 detected, that does not necessarily preclude that nothing unusual 

 happens in the developmental stages of the hybrid, particularly 

 at the time of fertilization and early cleavage stages. At any 

 rate it would not be at all surprising if something unusual did 

 happen to bring about the fifty-one chromosomes. The need of a 

 careful study of the chromosomes in the jack is obvious. A 



