GROWTH AND SENESCENCE. 1015 



dividual dies, we must believe that the protoplasm of the repro- 

 ductive elements is not subject to natural death, but has a self- 

 perpetuating metabolism which under proper conditions makes it 

 immortal. Weissmann* designates the protoplasm of the germ cells 

 as germ-plasm, that of the rest of the body as somatoplasm, and 

 inasmuch as the former continues to propagate itself indefinitely 

 under proper conditions, while the latter has a limited existence, he 

 concludes that originally protoplasm possessed the property of 

 potential immortality. That is, barring accidents, disease, etc., it 

 was capable of reproducing itself indefinitely. He assmiies, moro 

 over, that this property is exhibited at present in many of the sim- 

 pler forms of life, such as the ameba. This latter phase of his 

 theory has been the subject of much interesting investigation,! with 

 some contradictory results, but it has been shown (Woodruff) that 

 a specimen of Paramecium, isolated and kept in a varying culture- 

 medium during three and a half years, passed through 2000 

 divisions at an average rate of three in every 48 hours, without the 

 appearance of signs of senility. Later experiments by the same 

 author continued over five years indicated that the single cell 

 originally isolated "possessed the potentiality to produce similar 

 cells to the number represented by 2 raised to the 3029th power, or 

 a volume of protoplasm approximately equal to 10^°™ times 

 the volume of the earth." Such a result would indicate the essen- 

 tial correctness of Weissmann's view. An interesting contribu- 

 tion to the subject of growth has been made by RubnerJ upon 

 the basis of the energy factor. His estimates were made upon 

 data collected for man and the following mammaha, horse, cow, 

 sheep, pig, dog, cat, rabbit, and guinea-pig — and they bring 

 out the surprising fact that human growth constitutes a type 

 of its own, differing greatly from that shown by the other mammals 

 named. His conclusions are expressed in two general laws which are 

 founded upon calculations made upon these animals in the first 

 period after birth during the time necessary for doubling the weight 

 of the animal : First, the law of constant energy consumption. During 

 the first period of growth the total amount of energy necessary 

 for maintenance (metabolism) and growth, as expressed by the 

 heat value of the food consumed, is the same for all mammals 

 except man. To form one kilogram of animal weight requires 

 in round numbers 4808 Calories in food; while for man about six 

 times this amount is needed. Since the several mammals con- 



* Weissmann, "Essays vipon Heredity and Kindred Biological Problems"; 

 also "Germ-plasm" in the "Contemporary Science Series." 



t See Maupas, "Archives de zoologie experimental et generale," 6, 165, 

 1888; Gotte, "Ueber den Ursprung des Todes," 1883; Woodruff, "American 

 Naturahst," 42, 500; also "Archiv f. Protistenkunde," 21, 263, 1911. 



X Rubner, "Das Problem der Lebensdauer, " etc., Berlin, 1908. 



