CAUSES OF STERILITY IN THE MULE. IQ 



was ever observed after the synizesis period, and no chromosomes 

 ever seem to remain unpaired in the primary spermatocyte, with 

 the exception of the accessory which is always unpaired and 

 passes undivided to one pole or the other. 



In the mule the period of synapsis is extremely fascinating, 

 and the facts presented here will no doubt stimulate many 

 investigators in the study of hybrid sex cells. The author con- 

 siders himself exceedingly fortunate in being able to obtain 

 hybrid material which was in such excellent condition for this 

 investigation, especially since the hybrid in question is the off- 

 spring of two vastly different parents. Thus far, it appears that 

 there is no case on record among the vertebrates, showing that 

 this important step in the process of maturation of the sex cells 

 of hybrids has been fully and conclusively demonstrated; and 

 since this important phenomenon has never been clearly demon- 

 strated in a hybrid of this nature, and since this material given 

 to me by Dr. Kalkus appeared promising from the start, the 

 author spared no time and energy in making a careful and pro- 

 longed study of this particular phase in the maturation of the 

 sex cells of the mule. 



It might be stated at the outset that little fusion of the chro- 

 matin material takes place, and that there is no definite time for 

 such fusion. As soon as the various chromatin threads in the 

 network of the nucleus become well defined, there appear in- 

 dications of pairing of some of the threads. This process con- 

 tinues even after the network breaks up and the chromosomes 

 make their appearance (Figs. 8, 9, and 12). Fig. 8 represents an 

 early stage in which two or three of the threads had fused, and 

 a beautiful case of two long threads lying side by side almost 

 across the entire diameter of the large nucleus. Fig. 9 shows 

 a cell in which many more threads had fused and some are still 

 in the process of fusing. Fig. 10 shows a case where only a 

 small amount of fusion had taken place before the parts of the 

 chromatin network had become loose; but the process of pairing 

 seems to have continued in two or three instances regardless of 

 the fact that some of the chromosomes were in the stage of dis- 

 integration. Fig. ii represents a more advanced case, where 

 apparently a great deal of the chromatin material had fused 



