12 



BULLETIN 1121, U. S. DEPARTMENT OF AGEICULTUEE 



ALLOWANCE FOR AGE OF DAM. 



Another conceivable cause of differences among the experiments is 

 the percentage of litters produced by immature females. The per- 

 centage of first litters in each experiment is shown in Table 6. 



Table 6. — The first litters frovi immature parents (mated at one or two months of age) as 

 percentages of the total number of litters (1916-1919). 



Experiment. 



Per 



cent. 



i 



1 Experiment. 



Per 



cent. 



Experiment. 



Per 



cent. 



Experiment. 



Per 

 cent. 



2.... 



14.3 

 18.8 

 18.4 

 22.3 



39 



15.3 

 16.2 

 17.4 

 8.3 



CO. 



21.6 

 23.8 

 20.1 

 16.8 



Cl 



20.5 



13 



' other 



CA 



C2 



25.3 



32.... 



1 Inbred... 



AC 



CL 



25.4 



35 



1 B 



cc 



CG 



23.8 













Assuming that first litters are at a disadvantage, Experiment B 

 should be superior to the inbreds, Experiment CC should be practi- 

 cally the same, while the other crossbreeding experiments should be 

 inferior. However, as pointed out in Part I, the slight inferiority of 

 first litters found in the present stock of guinea pigs is almost wholly 

 a seasonal , complication. A majority of the matings have been made 

 in summer and fall under favorable conditions, bringing a majority 

 of the first litters in winter and spring, when conditions are apt to be 

 poor. It is found that on making proper correction for season, the 

 low records for first litters born when the female is about 6 months old, 

 as well as the similarly low records near 18 months of age, are brought 

 almost to the 12-month and 24-month levels. It may safely be as- 

 serted that the differences in the average age of the dams in the 

 various experiments are negligible as causes of differences in their 



records. 



ALLOWANCE FOR SEX. 



There is a slight difference between the birth weight of males and 

 females and a slightly greater difference in their weaning weights. 

 Since, however, the sex ratio (Table 7) in no case departs widely from 

 equality, it has not been deemed necessary to find separate averages 

 for the sexes. 



Table 7. — Sex of young born in each experiment, 1916-1919. 

 [Those of unknown sex were in general found dead and in bad condition.] 



Experi- 

 ment. 



Males. 



Fe- 

 males. 



Sex 

 un- 

 known. 



Total. 



Males 

 per 100 

 females. 



Experi- 

 ment. 



Males. 



Fe- 

 males. 



Sex 

 un- 

 known. 



Total. 



Males 

 per 100 

 females. 



? 



13 



32 



35 



514 

 350 

 237 

 429 

 195 



444 

 361 

 245 

 398 

 177 



40 

 29 

 6 

 6 

 10 



998 

 740 

 488 

 833 

 382 



115.8 

 97.0 : 

 96.7 ■ 

 107.8 

 110.2 



CC 



Cl 



C2 



CL 



CG 



B 



TotaL. 



297 



302 



94 



303 



317 



87 



17 

 10 

 1 



617 

 629 

 182 



98.0 

 95.3 

 108.0 



39 



183 



285 



186 

 277 



12 

 19 



381 

 581 



98.4 

 102 9 



other 



564 

 2,289 

 653 

 206 

 271 



567 

 2,192 

 647 

 202 

 289 



39 

 130 

 34 

 2 

 11 



1,170 



4,611 



1,334 



410 



571 



99.5 1 

 104.4 ' 

 100.9 

 102.0 



93.8 

 1 



Inbred.. 

 CO 



761 



762 



36 



1,559 



99.9 



CA 



AC. :.. 



5,341 



5,262 



272 



10, 875 



10L5 



