CAUSES OF STERILITY IN THE MULE. 35 



The destruction of the sex cells in the mule appears to be re- 

 stricted to the primary spermatocytes, where it seems inevitable. 

 The incompatibility of the two widely different plasmas contri- 

 buted by the parents renders completion of spermatogenesis in 

 this hybrid impossible and the animal remains sterile. The 

 process of deterioration sets is as soon as cooperation of the chro- 

 matin material becomes necessary in the primary spermatocyte. 

 By far the largest amount of destruction takes place in the 

 "spireme stage" before the chromosomes can make their ap- 

 pearance. Sometimes the nucleus in this stage becomes enor- 

 mously distended and the thin chromatin threads break up into 

 numerous fragments before any noticeable pairing takes place. 

 Then again, the fragmentation takes place after some of the 

 threads fuse (Fig. 10), or after the chromosomes make their 

 appearance (Fig. 44). 



The cytoplasm seems to decay first and large masses of naked 

 nuclei in various degrees of decadence can be seen in some of the 

 tubules. Small masses of chromatin material, pieces of threads, 

 cytoplasm, and cell wall, are also abundant in many of the 

 tubules. Occasionally small, naked, though well-defined nuclei 

 are seen. Whether these are formed from cast-off material like 

 the one shown in Fig. 24, or whether they are small nuclei 

 similar to those shown in Fig. 42, retaining their structure after 

 the cytoplasm had fallen to pieces is difficult to determine. 

 The latter suggestion can easily be conceived, and the former is 

 not unreasonable, since there is no logical reason to make one 

 believe that the expelled material invariably remains attached 

 to the cell. A much more legitimate assumption would be that, 

 since the material is cast off by the cell, it ordinarily leaves the 

 cell entirely and becomes free among the other material in the 

 lumen of the tubule. This assumption would tend to explain 

 further the extremely rare occurrence of cells similar to the one 

 shown in Fig. 24. 



Figs. 44-52 show various types of degenerating cells. Fig. 44 

 is in the late prophase, and Fig. 45 in the metaphase showing a 

 partly expelled mass of disintegrating chromosomes. Figs. 46, 

 48 and 49 show the collapsed condition of the cytoplasm and the 

 dense nature of the nuclei. The nuclei sometimes retain their 



