REPRODUCTION 



57 



Many hybrid animals produce litters considerably larger than those 

 produced by the A strain. Griineberg (6i) reports taking 19 healthy 

 embryos just short of term from one hybrid female. Gates (56) reports an 

 average size of 7.4 with a range of 2 to 12 for 106 litters in a random bred 

 strain. This is fairly typical for many strains. 



Table 2 

 Order of Litter and Litter Size in A Strain Mice (From Bittner) 



The number of corpora lutea formed at the time of the last mating is, 

 with possible rare exceptions, identical with the number of eggs ovulated. 

 This number is quite highly correlated with parity (order of litter) and with 

 weight of the mother, but only slightly correlated with age (88). It may be 

 used as an index of pre-natal mortahty. MacDowell (86) finds that 33.9 per 

 cent of the ova that come to maturity are not represented by living young at 

 birth. This is an average figure based on results from several strains; there 

 are considerable strain differences. Thus the dba strain shows a higher pre- 

 natal mortality than the C57 black strain (Fekete, unpublished data). 



There is evidence that mouse ova may split to produce uniovular twins, 

 and that these may come to term, but the phenomenon is certainly rare 

 (15, 27, 59, 109, 129). 



Sex ratio. — According to genetic theory, males produce equal numbers of 

 male-producing and female-producing sperm, so that, except for a possible 

 dift'erence in functional capacity of the two types of sperm, or a possible 

 selective effect of prenatal mortahty, the sex ratio at birth should be 1:1. 

 MacDowell and Lord (89) have recorded the sex of 106 litters of mice in 

 which the number born was no less than the number of corpora lutea, and 

 hence in which prenatal mortahty is ruled out. Their count showed 



