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[1] 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 material during any given period of development. It is often said that there is a "colossal increase of nuclear mass" but no increase in the protoplasm 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 rejuvenescence. However, in Crepidula and Fulgur among the gastropods and in Styela among ascidians there is no great change in the nucleusplasma 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 sji increase of nuclear substance, is to be found the cause of such rejuvenescence as may occur in these stages.
IV. Longevity, Senescence and Rejuvenescence
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 indefinitely in the ascendent the organism is potentially immortal. Such a condition is shown in Paramecium where Woodruff[2] 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 environmental.
Duration of assimilation conditions not merely body size, but also length of life. Very large animals are long lived and small ones are
