CAUSES OF STERILITY IN THE MULE. I'J 



horse. Even in cases where long threads occur the resemblance 

 is slight, for invariably the thread is made up of portions of 

 various thicknesses, while the spireme in the horse is of a more 

 uniform diameter. 



2. Synopsis. 



It appears that thus far there was no necessity for the paternal 

 and maternal chromosomes in the cells of the hybrid to cooperate 

 to any noticeable extent in functioning, and while some conflicting 

 tendencies may be in operation, each group mixed with the other, 

 seems to have gone on performing its functions normally. 

 Material from both parents undoubtedly prevails in the somatic 

 cells of the mule as it does in the sex cells. Yet these materials 

 in the somatic cells, coming from two vastly different animals, 

 do not in the least interfere with each other physiologically. And 

 while no somatic tissue of the mule has been studied, any sus- 

 picion of abnormalities, such as disintegration and decay for 

 example in the muscle cells, could hardly be substantiated by 

 our knowledge of the health, strength, endurance, and longevity 

 of these hybrids. This suggests that developmental processes are 

 also normal. It is highly probable, then, that since the physio- 

 logical and developmental processes are normal that the mechan- 

 ism of cell division during the process of development and growth 

 is also normal ; the same as it is in the spermatogonial cells where 

 perfect mitotic figures appear and all of the chromosomes divide. 



The real conflict ensues during the various stages of the primary 

 spermatocyte as is so plainly evidenced by the numerous ab- 

 normalities which occur in these cells. In fact the conflicting 

 tendencies are so great that the destruction of each cell is in- 

 evitable, and no spermatozoa are produced, causing the hybrid 

 to be sterile. Up to this stage the paternal and maternal plasmas 

 evidently retain their individuality and would undoubtedly 

 continue to do so were it not for the wonderful phenomena of 

 reduction, which necessitates a fusion of the chromatin com- 

 ponents of the germ cells at this stage of maturation, as is known 

 in many of the normal forms in which spermatogenesis was care- 

 fully studied. 



It is now a well-known fact that in maturation of the germ 

 cells a reduction of the ordinary number of chromosomes to 



