FERTILITY OF PARENT SPECIES AND HYBRIDS. 99 



out as recessives and that we should expect the same results from such 

 fertile hybrid males as with guinea-pig males, 14 of them were mated 

 with guinea-pig females. The microscopic tests showed that 21 of 

 their 22 sons were indistinguishable from a normal guinea-pig male. 

 The one exception (6^1524) was the son of an F4 male (cr506) and a 

 guinea-pig female (9186). The same sire and dam gave two other 

 sons (d'lS and cTlG) with many motile sperm. The exceptional son 

 had nothing except motile sperm in the epididymis, but they were 

 extremely few in number. I am informed by Dr. W. E. Castle that 

 other sons of fertile hybrid males and guinea-pig females likewise showed 

 signs of sterility. Fertility, however, appears to be obtained most 

 frequently from this class of matings, as the records show that 95.5 

 per cent of the sons of fertile hybrid males and guinea-pigs had many 

 motile sperm. In view of these facts, the hypothesis (that fertility in 

 the hybrids simply means eliminating 8 disturbing factors) can not be 

 maintained. There is strong evidence of segregation, but the case may 

 be complicated by other conditions, such as interaction of factors and 

 the like. 



THE SECONDARY SEXUAL CHARACTERS. 



Some observations on the secondary sexual characters were made. 

 Although not taken as statistical data, they were numerous enough 

 to be of value. Male hybrids of all classes showed the sex instinct. 

 In fact, I have never seen a single healthy male hybrid, sterile or fertile, 

 which did not attempt copulation. The hybrids fought with each other 

 for the possession of the females. How successful they were in copu- 

 lation is not known, but since the organs were morphologically similar 

 to those of a normal guinea-pig, it is probable that there were no diffi- 

 culties in this respect at least. There is good evidence that ejaculation 

 took place and that normal uterine plugs were formed from the clotted 

 mass, as in the case of any normal guinea-pig. It is well known that 

 severing the spinal cord will often produce an e j aculation. The method 

 of killing the hybrids was to sever the skull and axis by holding the 

 head, swinging the animal and suddenly arresting the motion. It wa& 

 noticed that in all cases an ejaculation took place if one then pressed 

 the groin, a clot forming almost immediately. This clotting or coagula- 

 tion of the semen, supposed to be due to a ferment, vesiculase (Marshall 

 1910), is common to both the hybrid and the guinea-pig males and gives 

 rise to the uterine plug in the female. More than 200 hybrid males 

 showed this peculiar reaction when properly stimulated. No hybrids 

 failed to show it if they were killed when adult. Hence it is almost 

 certain that they were physiologically potent in every respect, except 

 in the production of sperm. The accessory organs, including the 

 seminal vesicles and prostate glands, were always apparently normal. 

 The only differences noted were that sterile hybrids might have small 

 testes and a pale, small epididymis. 



