Reproduction ox Milk Diet 



507 



the above colonies on the whole milk diet. To date, one of these 

 rats has littered twice and the other two once, since being placed 

 on the milk diet; for two of these litters the sire must have been 

 one of the male rats raised on the milk diet, but the other two 

 litters may have been sired by a male placed on the milk diet 

 from a mixed ration. 



The litters born in this experiment died within a day or two 

 except in two cases. In one case the mother successfully nursed 

 4 rats on being given a yeast pellet 1 daily in addition to the milk. 

 These 4 rats are now growing rapidly on the milk diet without 

 yeast, but for a period of time were almost hairless. In the 

 other case the mother rat is at present apparently successfully 

 rearing a litter of 6 on addition of 0.2 gram daily of alcohol- 

 extracted 2 yeast, indicating that the deficiency in the milk is 

 not that of vitamin B. 



One inference which it has seemed permissible to draw from 

 MattilTs experiments is that the failure to secure normal growth 

 on milk only is due in part to an improper balance between the 

 food constituents. This inference does not seem to be substan- 

 tiated, however, by a fourth experiment carried out by us in 

 which a colony of 7 rats (3 males and 4 females) have grown 

 normally 3 on an "artificial" dry milk composed of 



Casein 18.7 



Lactalbumin 3.1 



Alcohol-soluble protein 0.5 



Butter fat 28.7 



Ether extract of alcohol-washed casein 0.5 



Protein-free milk 48.5 



This mixture contains the essential ingredients of cow's milk in 

 the approximate proportions as they are secured from whole 

 milk. To our surprise three of the females in this colony have 

 littered once and two have littered twice. 4 The first litter of 



1 The yeast was a dried whole yeast for which we are indebted to the 

 Northwestern Yeast Company, of Chicago. The amount given daily varied 

 between 0.2 gram and 1 gram and probably averaged 0.5 for the nursing 

 period. 



2 The dry whole yeast was extracted for 24 hours with hot 80 per cent, 

 alcohol in a Soxhlet type of extractor. 



3 A cage with wire bottom of one-quarter inch mesh was used. 



■* The second litters were sired by males on normal diet, it being neces- 

 sary to reduce the colony because of shortage of some of the ingredients of 

 the ration. 



