36 J- E. WODSEDALEK. 



normal shape but the chromatin material within fuses into a 

 continuous mass, which is often perforated by many vacuoles 

 (Figs. 46 and 49). Occasionally a single large vacuole appears 

 in the center of the nucleus leaving the chromatin material 

 arranged in a layer next to the nuclear membrane (Figs. 48 and 



52). 



Fig. 47 is a degenerate cell possibly in the late prophase stage 

 in which many of the chromosomes have fused into large spherical 

 bodies which remained scattered about in the cytoplasm. Fig. 50 

 shows a cell with an ill-formed nucleus and a mass of chromatin 

 material left out in the highly degenerated cytoplasm. Fig. 51 

 shows a cell in an advanced stage of destruction in all of its parts. 

 Fig. 52 shows a collapsed nucleus with a portion of naked cyto- 

 plasm. 



On account of the broadcast destruction of the primary sper- 

 matocytes in the early stages, a comparatively few are left that 

 attain the metaphase and early anaphase stages (Figs. 18-45) 

 when they, too, meet their fate; and the remaining few that 

 survive the anaphase succumb soon after, and no secondary sper- 

 matocytes nor spermatids, and consequently no spermatozoa 

 are formed and the hybrid therefore remains sterile. 



The invasion of tubules by wandering cells is not as pronounced 

 in the mule as in pigeon hybrids, as described by Guyer ('oo). 

 Even where the degenerative activity was most pronounced the 

 leucocytes were rare or absent altogether. Nor was there any 

 indication of the germinal cells themselves losing their cell walls 

 and characteristic appearance and becoming leucocytes, as some- 

 times appears to be the case in hybrid pigeons according to the 



same author. 



SUMMARY. 



1 . The parents of the mule are widely different as can be seen 

 from the comparison of the horse and the ass. 



2. The mule partakes of both the sire and the dam, but appears 

 to resemble the ass more than the horse, both in structure and 

 habits. 



3. The greatest difference seems to lie in the relative number of 

 chromosomes in the cells of these two animals. The horse 

 has thirty-seven and the mule fifty-one. This suggests that the 



