CAUSES OF STERILITY IN THE MULE. 3! 



impression that the spindle was a tripolar one, with two poles on 

 one side of the double plate and a single pole on the opposite 

 side, and that the single pole was cut off in the process of sec- 

 tioning. But a careful investigation shows that the cell is entire 

 and that no part of it was cut off. Guyer ('oo) represents a 

 similar case in hybrid pigeons, and describes it as a case in which 

 each fiber of the unusually loose spindle seems to terminate at 

 one end in a small centrosome-like dot or granule. 



Fig. 31 shows a tripolar spindle which is really a case of two 

 spindles fused together. All of the chromosomes are found on 

 the spindle and some are apparently in the process of division. 

 Fig. 32 shows another type of tripolar spindle and a group of 

 nineteen chromosomes, including the accessory, in the cyto- 

 plasm, corresponding to the group of chromosomes which are 

 sometimes expelled from the cell. Fig. 33 shows a degenerate 

 cell with three ragged spindles converging at a common point. 

 The accessory chromosome is off by itself and from it extends a 

 thread toward the point of convergence of the three spindles. 

 The cell is well along in the stage of decadence. Some of the 

 spindle threads are broken up and others seem to have fused 

 together. 



Fig. 34 shows a cell with three perfect spindles meeting at a 

 common conspicuous centrosome. The bivalent chromosomes 

 appear to be distributed among the three spindles. Fig. 35 

 shows an extremely rare type of quadripolar cell with five spindles 

 closely and symmetrically arranged, with three chromosomes 

 remaining loose in the cytoplasm. Fig. 36 shows a cell which is 

 the only one of its kind that was found and is no doubt the ana- 

 phase stage of a cell with a quintespindle arrangement similar to 

 that represented in Fig. 35. Eighty-five chromosomes, including 

 the accessory near the centrosome to the left, could be distin- 

 guished as represented. Six of the chromosomes in the central 

 spindle evidently had not divided. This in terms of univalence 

 would suggest that there is a total of ninety-one chromosomes 

 in the cell. The accessory remains undivided as it does in the 

 horse in the primary spermatocyte. Without the accessory, 

 then, there would be ninety chromosomes, which suggest rather 

 strongly that forty-five autosomes, of which five were presumably 



