88 GENETIC STUDIES ON A CAVY SPECIES CROSS. 



destroy any even remote chance of successful breeding. Furthermore, 

 at this period of investigation, facilities were available for mating the 

 males, and the need of a more rapid and expedient test was not felt. 

 The testes of some of these males were preserved for a later cytological 

 study. 



THE RESULTS OF ALL MICROSCOPIC TESTS. 



Our knowledge of the fertility of about two-thirds of the hybrid males 

 depends entirely on the examination of the contents of the epididymis 

 (see table 74). Out of a total of 483 males, 331 were tested in this 

 manner alone, and 102 males received both a breeding and microscopic 

 test. The total number of microscopic tests was therefore 433 (see 

 table 75) . The results of the microscopic examination in those animals 

 having both tests are given in table 76. For the sake of convenience, 

 all microscopic tests will be discussed together, thus giving larger 

 numbers from which to draw conclusions in table 75. The hybrids 

 are divided into four categories: with no evidence of sperm; with 

 evidence of any sort of sperm; with any motile sperm; and with many 

 motile sperm. A careful search made the first three classes easy to 

 differentiate, but one must admit that there are no sharp class lines 

 between the relative numbers of motile sperm. The classification 

 "many motile spermatozoa" means that the examination showed an 

 abundance of cells, all or practically all of which were motile sperm, 

 being the same condition v/^hich prevails in the guinea-pig (see table 75). 



(1) Hybrid males without spermatozoa. — ^Although the contents of 

 the epididymis were taken from several tubules at different levels, and 

 often from both testes, and at different times, some hybrids failed to 

 reveal any spermatozoa or any evidence of such in the form of disin- 

 tegrating flagella and the like. Such hybrid males, however, varied 

 widely in the nature of their contents. The early hybrids without 

 spermatozoa, such as the } wUd, usually showed a thin, clear, colorless 

 liquid in the epididymis almost devoid of all cells, but hybrids of late 

 dilute wild-blooded generations usually showed a thick, creamy liquid 

 rich in cells and cell detritus. The cells present were apparently 

 spermatogonia or spermatocytes, prematurely proliferated. The uni- 

 formity of the cells also differed, for some males had various kinds 

 of cells, while in others all or most of the cells were apparently alike. 

 In the later generations, the entire contents were often large, highly 

 refractive cells, possibly spermatids, inasmuch as cells of this type 

 were observed to have, occasionally, incipient tails. The tubules of 

 the epididymis in the i wild hybrids were thin and pale, but this con- 

 dition became less and less frequent in later generations. The pro- 

 portion of males without spermatozoa also gradually decreased. In 

 general, we may say that the j wild hybrids without spermatozoa 

 showed a thin, clear liquid with a few small cells; but later generations 



