2 J. E. WODSEDALEK. 



on in correlation with animal and plant breeding before any 

 adequate explanation of the results of hybridization can be ob- 

 tained, the study of the germ cells in hybrids themselves has thus 

 far been somewhat neglected. 



Guyer ('oo) in his excellent work on the spermatogenesis of 

 normal and hybrid pigeons says: "It is a remarkable fact that 

 no attempt has been made so far to investigate carefully the 

 spermatogenesis or ovogenesis of hybrid forms. In all the mass 

 of literature discussing or touching upon hybridism, so far as I 

 have been able to ascertain, there has been in no instance an 

 approach to a thorough study of the germ cells. Yet almost 

 every writer states that through the study of hybrids, we have 

 perhaps the best opportunity for gaining a clew to many of the 

 most vital points in the great problem of heredity. A number 

 of investigators have remarked that in certain instances the 

 anthers, ovary, or testes as the case might be, were defective, 

 and have let the matter go at that." 



It is even more remarkable now, since fifteen years have 

 elapsed and no further attempt has been made to investigate 

 carefully the spermatogenesis or ovogenesis of other hybrid 

 forms. Professor Guyer's work on hybrid pigeons which was 

 published in 1900, the same year in which Mendel's investigations 

 were rediscovered, so far remains as the only outstanding piece 

 of work on that particular subject. It is all the more remarkable 

 when we consider the fact that such a great deal of cytological 

 work has been done during the past fifteen years and that by far 

 the largest bulk of the work on hybridization has been done 

 during the same period. 



The mule is probably one of the best known hybrids for he is 

 raised in practically all parts of the civilized world. He has 

 been known for many centuries and has been used more or less 

 in Europe since the days before Christ, for Varro who wrote in 

 the first century B.C. refers to mules in Roman agriculture. 

 Sterility in mules has no doubt been a subject of discussion for as 

 many centuries but the real nature of the cause of sterility in 

 these hybrids has heretofore never been carefully investigated. 

 The spermatogenesis of the horse which was carefully worked 

 out at the University of Wisconsin Zoological Laboratories a 



