92 GENETIC STUDIES ON A CAVY SPECIES CROSS. 



hybrids with an abundance of motile sperm were actually fertile, while 

 10.2 per cent were sterile in breeding, a phenomenon which would not 

 happen with normal guinea-pigs. 



From this I conclude that the number and motility of the sperm 

 are not the only essentials for a real fertility, inasmuch as real fertility 

 in the last analysis must mean the capacity to fertilize eggs and sire 

 young. There are further reasons for concluding that the motile sperm 

 of hybrid males may be physiologically different from those of a normal 

 guinea-pig; for it often required much more time to obtain young 

 from the hybrid males, and the litters were unexpectedly small. In 

 129 litters from hybrid males, there were 238 young — an average of 

 1.84 per litter. The normal guinea-pigs produce about 2.4 young per 

 litter. Some hybrid males produced large, vigorous litters, and others 

 produced but few young after long mating. It was of course impossible 

 to tell what proportion of the motile sperm formed were qualitatively 

 complete in all essentials to perfect fertility; but undoubtedly some 

 male hybrids with many motile sperm lacked other indispensable 

 qualities, partly or completely. In addition, it may be stated that 

 sterility was not due to the absence of the secondary sex characters, 

 since all sorts of males, sterile or fertile, copulated and appeared 

 otherwise normal. 



THE INHERITANCE OF STERILITY. 



Two species, fertile under the same conditions, were crossed and 

 gave rise to sterility in the male hybrids. Some condition subsequent 

 to hybridization disturbed gametogenesis in the males, but did not 

 affect the females. The disturbing elements were carried and trans- 

 mitted by the females, however, for crossing these back to the male 

 guinea-pig gave sterile males again. After continued back crosses to 

 the guinea-pig, increasing signs of fertility appeared and eventually 

 completely fertile males were produced. The cause of the disturbance 

 had, to all appearances, segregated out. One can hardly refrain from 

 the thought that these fertile males segregated out in a Mendelian 

 sense, and that there were a number of physiological factors involved 

 and transmitted alternatively, the different recombinations of which 

 gave the various expressions of fertility and sterility. To be concrete, 

 had the sterility of the ^ wild males been due to one simple factor, or 

 to a group of completely coupled factors, or to disturbances between 

 one homologous pair of chromosomes at some stage of reduction, then 

 we should have expected 50 per cent of the I wild males to be fertile. 

 If the heterozygous condition of an allelomorphic pair, Aa, caused ster- 

 ility in the | wild males, but did not affect their sisters, then mating 

 these females back to the tame, aa, would give 50 per cent Aa + 50 



