NUTRITION AND GROWTH. 6$ 



experience, though limited, invites attention anew to the possible 

 nutritive functions of bacteria in the alimentary tract. Some of the 

 aspects of this problem are referred to in our earlier paper.* 



NUTRITION AND GROWTH. 



The criteria of adequate nutrition are quite different in the case 

 of growing animals from those applying to adults of the same species. 

 During the period of adolescence it is not sufficient to maintain a 

 condition of nutritive equilibrium and constancy of form or body- 

 weight. In this stage of an animal's existence there should be evi- 

 dences of development, and growth should manifest itself in a change 

 of size. The curve of growth, expressed in changes of body-weight, is 

 remarkably constant and characteristic for each species under the 

 ordinary conditions of nutrition and environment. The individual 

 values may at times fluctuate about a mean ; but in the majority of 

 cases the excursions from the average are not extensive. 



In Chart XXII are reproduced curves illustrating the average 

 normal rate of growth of the white rat, both male and female. The 

 statistics for two of the curves are taken from Donaldson,! whose 

 observations we have repeatedly verified in their general features. 

 A third curve on the same chart represents the results of our own 

 observations on the growth of the female white rat, regarding which 

 data are less abundant. It will be noted that the curves of growth 

 for the two sexes do not completely coincide in type. After an age 

 of 70 days, represented by a body- weight of about ioo grams, the rate 

 of growth is somewhat slower in the female than in the male. In- 

 deed, the females rarely attain the large weight and size exhibited by 

 the normal adult males of the same age, even in the case of animals 

 from the same litter. We gain the impression that our "breed" of 

 rats may in general be somewhat smaller than those measured by 

 Donaldson and his collaborators. At any rate, the data available 

 for statistical purposes are not very extensive and the curves here 

 presented must have only a provisional value until more numerous 

 measurements are made. In connection with certain of our experi- 

 ments it may be stated that "the effect of mating on the growth- 

 curve for the males can probably be neglected. "J In the case of 

 females, the effect of the bearing of young is, according to Watson, 

 "to render the mated rats slightly heavier than the unmated some 

 of the excessive weight being due to the larger amount of fat present 

 in the mated animals." Two charts (XXIV, XXV) are appended 



*Carnegie Institution of Washington, Publication No. 156, p. 3. 



fDonaldson: A comparison of the white rat with man in respect to the growth of the 

 entire body. Boas Memorial Volume, New York, 1906. 

 JCf. Donaldson: ibid, p. 8. 

 Watson: Journal of Comparative Neurology, 1905, xv, p. 523. 



