THE SIZE OF ORGANISMS 191 



volume of the protoplasm more than doubles, at the expense of the yolk, 

 between the 1-cell and the 24-cell stages, while the total nuclear volume 

 increases less than 1.5 times during this period. Jennings 22 has shown 

 that the rate of growth is numerically greater than I had stated if one 

 compares any stage with its immediately preceding stage, but of course 

 this criticism does not apply to the total actual growth of nuclear mate- 

 rial during any given period of development. It is often said that there 

 is a " colossal increase of nuclear mass " but no increase in the proto- 

 plasm during the cleavage stages of the egg; and correspondingly there 

 is said to be a great increase in the ratio of nucleus to plasma in the 

 cleavage period. Upon this supposed increase in the nuclear material 

 as compared with the plasma, Minot and Hertwig have based their 

 hypotheses that the cleavage of the egg represents a period of rejuvenes- 

 cence. However, in Crepidula and Fulgur among the gastropods and 

 in Styela among ascidians there is no great change in the nucleus- 

 plasma ratio during cleavage, and I believe that this will be found to 

 be generally true for other animals. On the other hand, there is a 

 considerable increase in the plasma at the expense of the yolk, during 

 the cleavage period in these animals, and in this fact, rather than in an 

 increase of nuclear substance, is to be found the cause of such rejuvenes- 

 cence as may occur in these stages. 



IV. Longevity, Senescence and Eejuvenescence 



Apart from accidental causes of death, longevity is determined by 

 the duration of the excess of anabolism over katabolism. If destructive 

 metabolic changes gain ascendency over constructive ones at an early 

 period the organism is short lived; if constructive processes are indefi- 

 nitely in the ascendent the organism is potentially immortal. Such a 

 condition is shown in Paramecium where Woodruff 23 has reared more 

 than 3,000 generations without conjugation and without loss of vitality. 

 These and other similar experiments have demonstrated the essential 

 truth of Weismann's doctrine that Protozoa are potentially immortal. 

 Woodruff found that the most important factors for maintaining vigor 

 are proper food and freedom from the poisonous effects of waste 

 products. In higher animals there is no doubt that both of these 

 environmental factors are important, but there are also other important 

 factors which influence length of life which are not entirely environ- 

 mental. 



Duration of assimilation conditions not merely body size, but also 

 length of life. Very large animals are long lived and small ones are 



22 Jennings, ' ' Nuclear Growth during Early Development, ' ' Am. Nat., 46, 

 1912. 



23 Woodruff, "Dreitausend und dreihundert Generationen von Paramecium, 

 etc.," Biol. Centralb., 33, 1913. 



