CAUSES OF STERILITY IN THE MULE. 2."] 



not be underestimated. The cell is undoubtedly a primary 

 spermatocyte in the telophase stage. The cytoplasm has failed 

 to divide. This is in accordance wiih all other observations 

 made on primary spermatocytes in this hybrid, since not a single 

 case was noted in which the cytoplasm showed even the slightest 

 indications of division or construction. 



Just exactly what the nature of the cell was in its metaphase 

 stage with regard to the arrangement of the chromosomes can 

 not be definitely stated. One thing seems fairly certain, and 

 that is that no division of individual chromosomes took place. 

 For the number of chromosomes can still be fairly accurately 

 determined since they had become only partially disintegrated. 

 A count of the chromosomes in the various parts of the cell 

 reveals a total of about forty-three, fourteen in the oblong 

 nucleus, ten in the spherical one, three in the cytoplasm, and 

 sixteen in the small spherical nucleus outside of the cell proper. 

 The total of forty-three shows that eight chromosomes are lacking 

 to make the total of fifty-one contributed by the last spermato- 

 gonial division. In accordance with the prevailing scheme of 

 synapsis in other primary spermatocytes this means that eight 

 of the forty-three chromosomes should be bivalent and thirty-five 

 univalent. In examining the various chromosomes with regard 

 to size, nine of them seem to be noticeably larger than the rest; 

 five are in the oblong nucleus and four in the large spherical one- 

 One of the nine is possibly the accessory, thus leaving the required 

 number of eight bivalent chromosomes. The three left behind 

 in the cytoplasm and the sixteen outcasts are all univalents. 



As to the origin of the two nuclei within the cell, several 

 possibilities suggest themselves. The possibility which appears 

 most tenable is that the cell in its metaphase stage resembled 

 the one represented in Fig. 32. It can be seen from that figure 

 that the cell possessed a large spindle, at one side of which was a 

 plate of chromosomes with threads extending only to one pole. 

 In another part of the cell is a group of about twenty chromo- 

 somes with no signs of spindle formation. It can be assumed 

 that the two nuclei shown in Fig. 24 were formed from two 

 masses of chromosomes resembling the two plates of chromo- 

 somes shown in Fig. 32, and that the nuclei were reorganized 



