6 J. E. WODSEDALEK. 



and extends some distance beyond the heels to the rear (Fig. 443). 

 Also, the shape of the foot of the ass, from above downwards, is 

 more or less cylindrical; and that of the horse is more or less in 

 the form of a truncated cone. 



"17. The teeth of the horse are more highly specialized than 

 those of the ass. . . . We can see in Fig. 643, that the gradual 

 lengthening of the crowns of the molar teeth is a well-marked 

 feature in equine evolution ; and from a study of the mechanism 

 of equine dentition, we learn, that for purposes of food-prehension 

 and mastication, the growth of the incisors must be proportionate 

 to that of the molars. Hence, we may assume that the crown of 

 the incisors, like those of the molars, have gradually lengthened 

 during the evolution of the horse; and consequently that the 

 acuteness of the angle made by the upper and lower incisors of 

 the horse of today increases with age, to a greater extent than 

 it did in the case of his ancestors. On turning to the domestic 

 ass, we find that in old donkeys (Figs. 448, 449 and 450), the angle 

 in question is greater in the horses of similar ages (Figs. 445, 446, 

 and 447). Consequently we may infer that the teeth of the 

 domestic ass are of an older type than those of the horse. Also, 

 the incisors of the donkey are relatively narrower than those of 

 the horse." In addition it might be said that the ears of the ass 

 are much larger and longer. 



All of the differences enumerated above are skin-deep, or 

 anatomical in nature. The present study has revealed another 

 difference, a cytological one, which is probably the most impor- 

 tant, for it lies in the finer structure of the cells of these animals 

 and no doubt is at the base of all of the differences perceptible 

 to the naked eye. It was learned that the cells of the horse con- 

 tain thirty-seven chromosomes (Wodsedalek, '14). The present 

 study shows that the cells of the mule possess fifty-one chromo- 

 somes. This would suggest that the cells of the ass possess about 

 sixty-five chromosomes, if nothing unusual happens in the de- 

 velopment of the hybrid, thus making the remarkable difference 

 of twenty-eight chromosomes between the cells of the horse and 

 those of the ass. This matter is considered at length in the latter 

 part of the paper. 



