ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 181 



It has been pointed out by Donaldson that we may assame the span 

 of life in the albino rat to be three years ; and that this is equivalent to 

 ninety years in man. On this assumption the rat grows thirty times as 

 rapidly as man. The early stages of development in the albino rat go 

 very slowly. In the growing foetus the rate tends to diminish with 

 age. The average weight of the foetus is greater in the horn containing 

 the smaller number. The diet of the mother appears to influence the 

 number of offspring. 



Relative Weight-changes in the Organs of Young Albino Rats.* 

 C. M. Jackson has made an experimental study of the changes iu the 

 relative weights of the difl'erent parts, organs, and systems of young 

 albino rats, kept at constant body-weight by under-feeding for various 

 periods. He finds that young rats may be kept at constant body- 

 weight for considerable periods by under-feeding, the amount of food 

 necessary for this purpose decreasing as the experiment proceeds. The 

 relative weights of head, trunk and extremities remain practically 

 unchanged. There is apparently a slight increase in the head, counter- 

 balanced by a slight decrease in the trunk and extremities ; but the 

 cbange is so slight that it seems of doubtful significance. There is 

 little change in the weights of the musculature and the viscera as a 

 whole. There is, however, a marked decrease in the weight of the 

 integument, and a marked increase in the skeleton. Thus, on the 

 low plane of nutrition in the young body maintained at constant 

 weight, the growth capacity appears weakest in the skin and 

 strongest in the skeletal system. This is in striking contrast with 

 the normal growth process of corresponding ages, during which the 

 musculature increases with relatively great rapidity, while the skele- 

 ton lags behind. The skeletal increase appears to involve the 

 ligaments as well as the cartilages and bones. It tends to proceed 

 along the lines of normal development, as indicated by decrease in 

 the water-content, and by formation and union of various epiphyses. 

 Another evidence of the tendency to normal development is seen in 

 the increased relative length of the tail, as compared with the body- 

 length. The teeth also continue to develop normally. 



The individual viscera may be classified in three groups : — 1. There 

 is, during the maintenance of constant body- weight in young rats, a 

 well-marked increase in the weights of the eye-balls, spinal cord, 

 alimentary canal (both empty and including contents), testes, hypo- 

 physis, and supra-renal glands. The sujira-renals undergo sexual dif- 

 ferentiation in weight (as occurs normally), but the hypophysis 

 apparently does not. 2. There is no marked change in the weight of 

 the epididymis. The liver is variable, showing a definite increase in the 

 earlier periods, but a decrease later. The luugs show a slight decrease 

 in the earlier periods, but not in the later. There is a well-marked 

 decrease in the weights of the thymus (" hunger involution ") spleen, 

 thyroid gland, and ovaries. When the organs are similarly grouped 

 according to degree of loss during chronic inanition in the adult, 

 many differences are found in comparison with the corresponding 



* Journ. Exper. Zool., xix. (1915) pp. 99-156 (4 figs.). 



April 19th, 1916 o 



