86 GENETIC STUDIES ON A CAVY SPECIES CROSS. 



to ascertain whether or not a further breeding test should be applied. 

 The value of the test is apparent, for out of 102 males tested micro- 

 scopically 43 were found to have very few or no motile spermatozoa 

 present and every one of them failed to breed after the most rigid 

 breeding test. On the other hand, 44 males which proved to be fertile 

 in breeding had an abundance of motile spermatozoa in every case. 



The microscopic test was simple and expedient. A male tested in 

 this manner was anaesthetized by etherization; the scrotum was thor- 

 oughly washed with 75 per cent alcohol, and dried; and the animal 

 was stretched on his back. A small incision, or a cut made with 

 scissors, about f-inch long, at the posterior end of the scrotum, exposed 

 the edipidymis. Several of the tubules were then transected with a 

 very small, sharp scalpel, and the hquid contents which collected 

 were placed on a cover-glass. The cover-glass was transferred to a 

 slide, on which a drop of physiological salt solution had been placed. 

 The cover-glass, slide, and salt solution were, to be sure, always kept at 

 bodily temperature . The slide was then examined under the microscope 

 and a careful record of observations was made. There were 433 males 

 of the different hybrid generations tested in this manner. In all cases 

 a record was kept, showing which testicle has been used for operation. 

 For the sake of convenience the left testicle was always used. Bilateral 

 tests were made in enough cases to show that either testicle would give 

 the same result; but such tests were made only after a thorough breed- 

 ing test or with surplus animals, for transection of the epididjrmis on 

 both testicles might make an animal sterile in breeding, although 

 potentially fertile. The wound was covered with iodoform and healed 

 completely in a week. 



In order to exclude any possibility of varying tests on one and the 

 same animal under different conditions, over 100 males were retested, 

 both on the left side and on the right, in summer and in winter, and 

 in good condition as well as in very poor condition. The second and 

 third tests always gave the same results as the first, with the following 

 exceptions : the cellular contents of the epididymis were always of the 

 same character; but it must be stated that 3 males showed immotile 

 sperm on the first test, but motile sperm on a second test some months 

 later.* I am fully satisfied that the difference was due to my own early 

 inexperience. Thereverseneveroccurred,forwhenasecond testshowed 

 immotile sperm after a first test had shown motile sperm, I could always 

 locate the difficulty and immediately produce a repetition of the first 

 results. Hence, I am inclined to believe that these 3 aberrant animals 

 originally had motile sperm, and had simply failed to show it because 

 the temperature of the slide was too low or because evaporation had 



*The term, sperm, used to avoid frequent repetition of the cumbersome term, spermatozoa, 

 will be clear from the context. 



