FERTILITY OF PARENT SPECIES AND HYBRIDS. 89 



showed increasing numbers of cells and more highly differentiated cells. 

 The transition was gradual. It is probable that the cells were incom- 

 pletely matured germ cells. 



The ^ wild male in tables 75 and 76 (cr70) was examined from a 

 histological preparation of the testis made by Dr. W. E. Castle. 



(2) Hybrid males with spermatozoa. — All classes of hybrids, from the 

 Fo generation on, contained some individuals showing spermatozoa. The 

 difference between individuals was great, both in respect to quantity 

 and character of sperm. Two j wild males showed a few very imperfect , 

 non-motile sperm mixed with a few of the usual cells. Twenty-two 

 I wild males likewise showed sperm, but in greater numbers and some- 

 times motile. The percentage showing sperm gradually increased, 

 as would naturally follow^, since the percentage without sperm gradually 

 decreased. In the Fe generation (-gV wild) about 96 per cent showed 

 sperm. The F7 generation showed sperm in 87 per cent of the cases; 

 but since the total number w^as only 15, the results are subject to a 

 valid objection. I am inclined to believe that larger numbers would 

 have given a perfect series. When few sperm were present, only few 

 other cells might be present also, as in the j wild. In the later genera- 

 tions, if the sperm were infrequent, there usually was an abundance 

 of other cells. Moreover, the sperm present varied in motility or 

 might be misshapen or normal. If we simply consider the presence 

 of any kind of sperm, table 75 shows that the percentage of males with 

 sperm gradually increased as the wild blood became more dilute. The 

 proportion with many sperm also gradually increased, while the pro- 

 portion with few sperm decreased. 



(3) Hybrid males with motile spermatozoa. — Hybrids showing sperm 

 did not necessarily show motile sperm. Rarely a hybrid would have 

 practically nothing but sperm, yet all of them immotile. Such animals 

 w^ould of course be sterile. In other cases hybrids showed only few 

 sperm mixed with the usual cells, but all the sperm were motile. The 

 variations between these two classes were continuous. The percent- 

 ages showing any motile sperm whatever increased from 16.33 per 

 cent in the \ wild to 86,67 per cent in the y4^ wild; and conversely, the 

 proportion with no motile sperm gradually decreased in each genera- 

 tion after the \ wild. 



(4) Hybrid males with many motile spermatozoa. — -Males having but 

 few motile sperm could not be bred successfully. This may have been 

 due simply to the fact that there was less chance for a spermatozoon 

 to reach an egg. I am inclined to believe, however, that mere abund- 

 ance of motile sperm is not the only essential to fecundation, as will 

 be shown later. It may well be that hybrids producing motile sperm 

 sometimes fail to produce sperm qualitatively adequate. The greatest 

 success in breeding was obtained wdth males showing an abundance 

 of motile sperm. By abundance or "many motile sperm," as used in 



