MODIFICATION OF THE GERM-CELLS IN MAMMALS 147 



mal guinea-pig, yet such a phenomenon is not very common, 

 although in the alcohoHc lines it is frequently observed. We 

 shall consider this process below, the only point of interest here 

 being its effect on the size of the litter. 



The exactly reversed percentages of individuals born in large 

 and small litters in the normal and alcoholic lines, as shown by 

 the table, may indicate that one-third of the animals in alcoholic 

 lines that are born in litters of one or two were originally in litters 

 of three, four or five. For example, the normal lines have in all 

 well over 12 per cent more animals born in litters of four or five 

 than in litters of one or two, and the alcoholic lines have over 

 12 per cent more in litters of one or two than in litters of four 

 or five, and this 12 per cent probably has been thrown from the 

 larger into the smaller litters on account of early abortions and 

 absorptions which occur in the former. The too frequent oc- 

 currence of small litters is undoubtedly indicative of not alone 

 an actually low productivity, but a very early prenatal mortality. 



Another occurrence also partly due to an early prenatal mor- 

 tality is the failure of a mating to produce a result. No doubt 

 in rare cases fertile guinea-pigs may be mated during the heat 

 period of the female, as these have been, without a following 

 conception. In the normal lines four out of eighty-eight matings, 

 or 4.54 per cent, failed, giving negative results, while in the 

 alcoholic lines three times as many matings failed, and very 

 probably this excess represents those cases in which not only a 

 part of the litter is lost through an early prenatal mortality, 

 but the entire litter is destroyed. Of course, some cases of 

 actually infertile matings are also represented. 



By this 'early prenatal mortality' is meant the absorption or 

 loss of an embryo before it is of sufficient size to be detected on 

 carefully feehng the uterus through the body wall of the mother. 

 With experience an embryo eight or ten days old may be de- 

 tected by an external examination of the uterus. Through our 

 routine examination of the females after being with the males 

 for one month, any embryo lost after this time will have been 

 discovered and is definitely recorded in the third horizontal space 

 of the table. If absorption or early abortion of one or more 

 embryos -in a litter may actually be observed to occur after as 



