CAUSES OF STERILITY IN THE MULE. 15 







study of the sex cells in both sexes of the hinney, as well as those 

 of the female mule and she-ass or jennet, would also add materi- 

 ally to this immensely interesting problem of sterility of the horse 

 and ass hybrids. 



Fig. 3 represents a polar view of the metaphase stage of a 

 spermatogonial cell. The fifty-one chromosomes are distributed 

 throughout the entire plane of the equator and the larger ones, 

 as a rule, are arranged along the edge. The heart-shaped ac- 

 cessory can be readily distinguished not only on account of its 

 shape, but also on account of its large size. It is invariably 

 at or near the edge of the equator. Fig. 4 is a side view of the 

 spindle and a representation of one of the many perfect spindles 

 found in this stage. There are no scattered chromosomes to be 

 seen; each enters the equatorial plate and divides. The fact is 

 further evidenced by the perfect divisions which follow. During 

 the anaphase stage the chromosomes move to the opposite poles. 

 Figs. 5 and 6 show that there are no stragglers among the chro- 

 mosomes. The accessory which also divides in this stage, can 

 practically in all cases, be detected on account of its larger size. 

 In the telophase stage when the chromosomes are at the opposite 

 poles the cytoplasm begins to constrict in the center. Soon the 

 chromosomes become ragged and later disintegrate while the 

 nuclear membrane makes its appearance. Following the last 

 spermatogonial division the cell enters into the period of great 

 growth and represents the early stage of the primary spermato- 

 cyte. 



PRIMARY SPERMATOCYTES. 



i . Early Propliase. 



The most important phase of this problem lies in the primary 

 spermatocytes. These cells are the result of the last spermato- 

 gonial division as in normal forms, and while their behavior is 

 always interesting and more or less difficult to analyze in any 

 case of spermatogenesis, it is especially so in this hybrid. From 

 the time of the appearance of these cells until the period of their 

 final destruction one is confronted with numerous fascinating 

 categories. 



During the early prophase the primary spermatocytes resemble 



