12 GENETIC STUDIES ON A CAVT SPECIES CROSS. 



Growth. — The wild species was observed to grow more slowly than 

 the tame guinea-pig. Hence weights of the wild, the hybrids, and the 

 tame were taken at frequent intervals and recorded. The animals 

 were weighed at birth, or shortly after, and then each week until the 

 nature of the individual growth curve was established. After that 

 they were weighed at less frequent intervals until they died or were 

 killed. This method also afforded an opportunity to keep strict watch 

 on the health of each animal, for a sharp, unexpected drop in weight 

 indicated sickness, fighting, or some other disturbing cause. Sexually 

 mature females were weighed immediately after parturition, in order 

 to ehminate the error due to a varying number of fetuses. 



Skeletal dimensions. — Just as the wild animal is smaller in total size, 

 so its individual bones were observed to be shorter and more slender 

 than those of the tame. The skull, lower jaw, right scapula, right fore- 

 leg, and right hind-leg of such adults as died were saved for further 

 observations. Whenever a hybrid reached maturity and could no 

 longer be used for other purposes, it was killed and the bones similarly 

 saved. A careful examination of the growth curve and the bone 

 sutures showed that guinea-pigs and hybrids are of full adult size when 

 15 months old. Measurements, of which a detailed account is given 

 later, were made and tabulated. 



Fertility. — The fertility of the wild, hybrid, and tame females was not 

 uniform. Records of the size of each litter were kept, from which 

 averages could be calculated. The wild males were fertile in captivity, 

 but their \ wild hybrid sons and their ^ wild grandsons were sterile. 

 The problem immediately suggested itself: how great must be the 

 blood dilution, or for how many generations must the hybrid females 

 be crossed back to the guinea-pig, before producing fertile males? 

 The numbers of males to be tested increased to such an extent that 

 facilities were lacking to test their fertility by mating them to females. 

 Furthermore, it is well known that a male may be potentially fertile, 

 but fail to show it because of some physiological state, such as extreme 

 emaciation from sickness, or through the sluggishness of obesity. 

 Another method was devised. It was observed from many cases that 

 the breeding test was negative whenever a male lacked spermatozoa in 

 the epididymis or when these spermatozoa were few, degenerate, or non- 

 motile. On the other hand it was found that fertile males invariably 

 have many motile spermatozoa in the epididymis. Examination of the 

 sperm content of the epididymis therefore affords a clear index of 

 fertility. The examination is readily accomplished by placing a drop 

 of the contents of the epididymis in normal salt solution at a bodily 

 temperature and examining it under the microscope. An operation of 

 this sort performed on one side of the body only does not preclude 

 subsequent breeding of the animal operated upon. 



