CHANGES IN COMPOSITION OF BODY OF RAT 31 



increases very rapidly during the first week and then at a some- 

 what slower rate. This change in the fat content is seen whether 

 the fat is considered as a fraction of the moist weight or of the 

 dry solids. This very rapid increase of the fat during the lac- 

 tation period seems to be accounted for by the very high con- 

 tent of the fat in the milk. The diminution of the fat, after 

 the lactation period is over, is very noticeable. 



This may be partly due to the rapid formation of the support- 

 ing system or tissues in general or, more probably, to an in- 

 creased metabolic activity. It must be stated that so far as 

 our experience goes, the rats are seldom fat as the age ol five or 

 six weeks. 



3. Organic extractives. All water soluble substances from which 

 the salts have been removed are here called the organic extrac- 

 tives. Since the organic extractives are mainly intermediate 

 metabolic products of the various organs and tissues, we should 

 expect their variation to be more or less in conformity with those 

 of the residue. This expectation is well realized since they 

 increase steadily as the body weight increases (see per cent in 

 terms of moist weight). When this group is considered as a 

 percentage of the dry substance, it is found to be highest at 

 birth and then to decrease towards the end of the lactation pe- 

 riod, after which there is another steady increase. In general 

 therefore its variation is related to the variations of the residue, 

 and is an index of growth and activity. 



4. Scluble salts. The salts so designated were obtained from 

 all the extractives made both with water and with alcohol. 

 The soluble salts exist in the body probably either dissolved in 

 the various body fluids or loosely combined with, various tissues. 

 The salts, considered as a percentage of the moist weight, are 

 nearly constant and the value is also the same as that for the 

 milk (stomach contents). I am unable to explain this curious 

 phenomenon. The soluble salts as a fraction of the dry sub- 

 stance decrease slowdy but steadily from birth onwards. 



5. Fixed salts. The solids from which fat, organic extractives 

 and soluble salts had been removed were incinerated and the 

 ash thus obtained is here called the fixed salts. These salts 



