﻿BOTANICAL GAZETTE 



ened. The essential cause of senescence is this deposit of colloidal material 

 in the bed of the living stream, a deposit which becomes optically visible in 

 the progressive differentiation of the protoplasm. This simple principle of the 

 guiding of the living stream by its erosions and sediments is at the bottom not 

 only a senescence and differentiation of cell protoplasm, but of all that differ- 

 entiation of the organism in embryonic development, which results finally, as 

 the living stream flows on, in the considerable chemical differences between 

 different organs and in the gradual and necessary aging of the individual. 

 Thus from a very simple germ a very complex organism is evolved, and quanti- 

 tative, not qualitative, differences in rate of metabolism produce eventually 

 a differentiation in kind of metabolism. 



The work is the outcome of the discovery of a particularly favorable material 

 for the study of the problem of senescence and rejuvenescence, together with 

 the discovery of a method of testing the degree of aging or youth. The material 

 discovered is the flatworm Planaria and related species. These animals often 

 do not reproduce sexually, but detach a part of the body which undergoes 

 dedifferentiation and rejuvenescence to a young animal. Such pieces emerge 

 from encystment young animals. In them, therefore, there is a regular series 

 of senescences and rejuvenescences taking place in a brief time, and experiments 

 may be tried on the control of the process to determine its nature. From these 

 forms one can interpret the similar phenomena shown by other organisms. 

 The method of study consists in the discovery of several simple ways of deter- 

 mining the degree of youth, or, in other words, the degree of metabolism, for 

 the metabolism of the young is keener than that of the old. One method is 

 by the use of potassium cyanide. The susceptibility to this poison is a direct 

 measure of the speed of metabolism, if animals of the same kind but different 

 ages are compared. In addition to potassium cyanide the susceptibility to 

 anesthetics, or the power of acclimatizing to very weak doses of anesthetics 

 such as alcohol, also furnishes a criterion of youth or age, or rather of youthful 

 metabolism. These methods have been controlled by the direct measurement 

 of the amount of carbon dioxide exhaled per gram of tissue as determined by 

 Tashiro in his biometer, and these measurements have completely confirmed 

 the observations on the cyanides. By observing the animals in cyanide solu- 

 tions, it is found that the parts of the animal which have the higher rate of 

 metabolism die first, the death changes being very clear and easily perceived. 

 It became possible thus to study the rate of metabolism in different parts of the 

 animal and under various conditions of age, temperature, after injury, etc., 

 and a large number of very striking and instructive experiments on many differ- 

 ent forms have been recorded. Chapters 3, 4, 5, and 6 of Part II deal with the 

 method, with the phenomena of the susceptibility to cyanide, and the rate 

 of metabolism occurring in the reconstitution of new organisms from pieces 

 of Planaria, whether these pieces are sexual cells, or cysts, or pieces cut 



