CAUSES OF STERILITY IN THE MULE. 25 



contents of the cell. The cytoplasm of these cells was invariably 

 degenerated ; the remains being closely assembled about the large 

 clump of chromosomes, leaving the periphery of the cell clear 

 in appearance. The cell wall, however, retained its normal out- 

 line up to this stage of degeneration, which 'seems to suggest 

 that the clear space was occupied by a fluid. While there were 

 indications of spindle threads more or less pronounced in some 

 cells, many of the cells showed no signs of the presence of such 

 threads (Fig. 18). In such cells they either disappeared or had 

 never been formed. It is a curious fact that such type of cells 

 usually appeared in large numbers in the same locality and that 

 only several tubules bearing them were found, though occasionally 

 single individuals of that type were found in other tubules. The 

 heart-shaped accessory could usually be seen somewhere off by 

 itself (Fig. 1 8). 



Figs. 19-21 show a common type of freak in which the spindle 

 is bipolar and a number of chromosomes did not find place in the 

 equatorial plate but remain scattered about, either somewhere on 

 the spindle or in the cytoplasm. Figs. 19 and 20 each show the 

 large accessory at one pole. In Fig. 21 the accessory could not 

 be easily detected. 



Fig. 22 shows an uncommon type of unequal division in which 

 five large chromosomes have passed over to one pole, while about 

 thirty-six remained in the original place. This indicates that 

 the total number of chromosomes in that cell is forty-one and 

 that ten of them, in view of the previous evidence and discussion, 

 must necessarily be bivalent. Each of the five at one pole on 

 account of their size appear to be bivalent. One of them, in 

 which the two parts have not fused along their entire lengths, 

 obviously shows its bivalent nature. Further evidence that 

 these five are double can be based on the fact that only about 

 five of the chromosomes in the big group appear to be double. 

 This shows then that no chromosomes have divided, but that 

 five bivalent ones were pulled over to one pole in their entirety. 

 Each of these groups without further division would in all prob- 

 ability, if decay did not set in too early, form a separate nucleus. 

 And it can be safely assumed that the binucleated condition of 

 cells sometimes seen in this tissue, in which one nucleus is much 

 larger than the other, was brought about in this fashion. 



