296 THE UNIVERSITY SCIENCE BULLETIN. 



In the case of the mule and horse twins, it might be sup- 

 posed that the presence of a mule foetus might in some way 

 affect the development of a horse foetus, and accordingly some 

 abnormal characters appear in the horse (Lillie, '17). So far 

 as I can see, no such characters are present. From the photo- 

 graphs that were taken (figs. 1-3), one can not detect mule 

 characters. In figure 3 the hair is long, due likely to winter 

 conditions. The photos of the mule show it likewise to be a 

 normal animal. 



The present occurrence of a mule and a horse as twins is 

 probably what might be termed a case of superfecundation 

 rather than superfoetation, since fertilization of both eggs 

 could have taken place only at one period of heat. We have 

 reason to think this from the fact that only one breeding to 

 each sire occurred, and these breedings at a ten-minute inter- 

 val. Mumford, in his text "The Breeding of Animals," gives 

 seven examples of what he terms superfoetation. Six of these 

 resulted in twin colts, one of which M^as a mule, the other a 

 horse ; but they resulted from fecundation at a second or later 

 period of heat. One of the colts, accordingly, was usually 

 smaller or less well developed than the other. In two of these 

 cases both died at birth or soon after ; in one case he does not 

 say; in two cases one twin only died; while in one case (his 

 plate II) both evidently lived. From the number of cases 

 cited by Mumford, it may be seen that the occurrence of mule 

 and horse twins is not such a rare phenomenon as one might 

 expect. The facts in the case here presented and in the other 

 cases probably indicate that such twins may be produced, not 

 only by fertilization during pregnancy at a recurring period 

 of heat, as Mumford's cases seem to be, but also as a result of 

 the appearance of two ova in the uterus at once. 



Newman, in his book "The Biology of Twins," p. 95, says: 

 "At the present time, I have no reliable evidence of twinning 

 in horses, but it is highly probable that this group offers no 

 exception to the general rule that all mammals normally pro- 

 ducing but a single offspring at a birth may have twins." The 

 present case and those of Mumford, I believe, are now good 

 evidence of twinning in the horse, and twinning in the case 

 here described is shown to be of a hereditary nature. 



