CAUSES OF STERILITY IN THE MULE. 23 



in their fusion. Frequently two threads, which have some affinity 

 for each other, and lie in a position parallel to each other and are 

 not too taut, fuse along the central portion with the ends remain- 

 ing in the shape of V's, because the two threads are attached at 

 each end some distance apart. At times when parts having an 

 attraction for each other are so arranged as to form an acute 

 angle, fusion begins at the point of the angle and continues a 

 short distance, leaving a V at one end. The ends remain unfused 

 at that point, until the network breaks up into chromosomes, 

 when the temporary connections of the two chromosomes in 

 question, too, are severed, and thereby complete their fusion. 



There is also considerable evidence that actual migration for 

 purpose of fusion of some parts of the net-work takes place before 

 the connections are entirely severed, and even before there are 

 any noticeable indications of such connections separating, for 

 frequently we find a bivalent thread with both ends still attached 

 to other threads. 



3. Abnormalities in Mitosis. 



The abnormalities in mitosis bear a striking resemblance to 

 the abnormalities found in the sex cells of hybrid pigeons by 

 Guyer ('oo). In speaking of abnormalities in pigeons, Dr. 

 Guyer says: 'The abnormalities in mitosis are in the nature of 

 multipolar spindles and asymmetrical division and distribution of 

 the chromosomes (Figs. 28-39). These may exist independently 

 one of another, or both may occur together in the same cell. 

 They are more pronounced in sterile birds but may at times be 

 seen in the fertile forms. In very many of the division of primary 

 spermatocytes one or the other, or both of these phenomena are 

 seen. It is a curious fact that the multipolar spindles seem to 

 be confined largely to the primary spermatocytes, and one is 

 prompted immediately to associate the fact with the pseudo- 

 reduction or formation of bivalent chromosomes which occurs 

 normally at this stage of spermatogenesis. The irregularities 

 in chromatin distribution are also seen for the most part in the 

 primary spermatocytes." 



The abnormalities in the mule are also in the nature of multi- 

 polar spindles and there are attempts at asymmetrical division 

 and distribution of the chromosomes; but in addition to these 



