CAUSES OF STERILITY IN THE MULE. 2Q 



a mass of chromosomes had evidently regenerated in this cell. 

 That the cell had begun to degenerate is evidenced by the 

 presence of large vacuoles and other irregularities in the cyto- 

 plasm. The signs of decay always seem to be manifest in the 

 cytoplasm first and then in the chromatin. 



The fact that more cells of exactly this type could not be 

 identified is no doubt due to decay which almost invariably sets 

 in before the aim of the cell is accomplished, as will be pointed 

 out later. It is evident that at least some of the other cells 

 seemed to act in the same direction but succumbed before the 

 reorganization of the nuclei could take place. Then, too, it 

 seems reasonable to believe that the isolation of a particular 

 group of chromosomes, as a necessary preliminary step to the 

 function of expulsion, depends largely on the relative arrangement 

 of the linin strands representing such a group of chromosomes in 

 the early stages of these cells. According to our knowledge of 

 the extreme variability in the structure of the linin strand net- 

 work, such a favorable arrangement of the unwelcome chromatin 

 material can possibly be regarded as being accidental. 



Judging by the appearance of the chromosomes, the author is 

 inclined to think that the material expelled by the cells is largely 

 that which was contributed by the mother of the hybrid; and 

 while the number of chromosomes so expelled varies in the differ- 

 ent cases, nineteen chromosomes including the accessory which 

 is the number presumably of maternal origin were found in a 

 few instances. There were never more than nineteen chromo- 

 somes in the expelled group. This, however, is only a matter of 

 suggestion, for the situation can not possibly be cleared up until 

 a careful study has been made of the sex cells of the ass. 



Fig. 25 shows a type of cell in which there is one' large spindle 

 and a bunch of chromosomes forming a small plate located at 

 right angles to the large one. It can be seen that a number of 

 threads radiate from the chromosomes in the small plate and 

 that some of them extend to the poles of the large spindle. The 

 large chromosome in the upper part of the spindle is the accessory. 

 Fig. 26 shows a peculiar quadripolar spindle. Most of the chro- 

 mosomes are in the plate of the large spindle and many of them 

 show signs of division. A small group of chromosomes is located 



