CAUSES OF STERILITY IN THE MULE. 1 1 



cytoplasm can often be seen near the nucleus. This is in all 

 probability the idiozome. These cells have definite walls which 

 mark them off clearly from the neighboring cells. They are 

 almost spherical and have the appearance of smear cells, a con- 

 dition presumably due to the fact that they are not crowded. 



As growth proceeds all parts of the cell increase in size. When 

 the maximum size is reached, dense masses of chromatin make 

 their appearance which become more and more defined until the 

 chromosomes of various shapes and sizes are formed. Some- 

 times the chromosomes are so close together that an accurate 

 count is impossible; but more frequently they are sufficiently 

 separated so that many definite counts were possible. 



There are fifty-one chromosomes found in the spermatogonia 

 of the mule. Fifty of these are the ordinary chromosomes or 

 autosomes, and one is the accessory which is larger than the others. 

 Fifty-one chromosomes is fourteen in excess of the number found 

 in the spermatogonial cells of the horse which has, at least in 

 some breeds, a total of thirty-seven, thirty-six autosomes and 

 one accessory (Wodsedalek, '14). 



A considerable amount of time was spent in an attempt to 

 ascertain which of the chromosomes are of paternal and which of 

 maternal origin, or in other words which were contributed by the 

 jack and mare respectively. And while the author has obtained 

 some fairly definite ideas in regard to the individuality of the 

 chromosomes in the mule, he does not feel that any positive state- 

 ments in regard to the matter can be made until the sex cells of 

 the ass will have been thoroughly investigated. 



It is quite certain, however, that the accessory chromosome 

 retains its individuality, for its appearance in the mule is identical 

 to that of the accessory of the horse. The mule no doubt re- 

 ceived the accessory from the mare in the same manner as it is 

 contributed by the dam to her male offspring in the horse, in 

 view of our knowledge of the behavior of this body in the sper- 

 matogenesis of the horse (Wodsedalek, '14), and particularly 

 according to our acknowledge of the behavior of the accessory 

 chromosomes in relation to sex determination in another mammal, 

 the pig (Wodsedalek, '13). In regard to the other chromosomes 

 it is definitely known that in the horse they vary considerably 



