120 P. B. SIVICKIS. 



egg capsules may be found in summer, but among the plants only 

 the smaller, younger animals have been collected. Stocks are kept 

 in the laboratory for three or four weeks before using experi- 

 mentally so that uniformity in nutritive conditions may be approxi- 

 mated. They are fed three times a week with ground and washed 

 beef liver, as described by Hyman ('20). 



The work in this laboratory with planarians has demonstrated 

 that in a stock of animals collected at one time, kept under as 

 nearly as possible identical conditions of temperature, nutrition, 

 water supply, etc., size is the best criterion of physiological condi- 

 tion, and particularly of physiological age, as indicated by suscepti- 

 bility and respiratory rate, which can readily be applied to the liv- 

 ing animals in the selection of material for experiment. Unin- 

 jured animals of the same size from such a stock show a high 

 degree of uniformity in susceptibility to chemical and physical 

 agents and in rate of respiration, as shown by the work of Child, 

 Hyman, Behre, and Buchanan, and are more alike physiologically 

 than material selected on any other basis thus far discovered. In 

 these animals the amount of growth, whether rapid or slow, and 

 consequently the size of the individual, is a far more exact measure 

 of their physiological age, and so of their susceptibility and rate 

 of respiration, than the length of time they have lived as indi- 

 viduals (Child, '15a, Chap. IV.; Hyman, '19 C). 



Since the experiments recorded in this paper are all mass ex- 

 periments — i.e., with numbers of individuals — the material for each 

 experiment is selected on the basis of size. Such standardization 

 of material is necessary for the attainment of definite results 

 which can be predicted and controlled and it makes possible pre- 

 diction and control to a high degree. In the course of the work 

 experiments were performed with standardized material from 

 general stocks collected and kept as described above, from stocks 

 composed of animals hatched from eggs in the laboratory, and 

 from stocks grown from cut pieces. 



The experimental data presented below concern chiefly respira- 

 tion, susceptibility, and head frequencies — i.e., the frequencies of 

 occurrence of the various forms of head in the reconstitution of 

 pieces in relation to level of body, length of piece, and physiological 

 age of animals. Some experiments on modification and control of 



