270 AGE, GROWTH, AND DEATH 



4. Growth. The cell retains permanently a portion of the 

 matter drawn in by the vortex. 



5. Multiplication. The cell cannot grow beyond a certain 

 limit, but instead of further enlargement it divides. (The bud- 

 ding of Infusoria is only a peculiar form of cell division.) 



6. Senescence. With each successive generation of cells the 

 power of growth diminishes. Were this otherwise, the growth 

 of each individual at any given time would be in geometrical 

 progression. This loss of power I term senescence. 



7. Rejuvenation. The effects of senescence are overcome by 

 some of the cells separating in character from the rest, and 

 giving rise to peculiar bodies, the eggs and spermatozoa. A new 

 cycle of cell generations is thus formed. In each cycle there is a 

 slow senescence terminating in the formation of a new cycle by 

 the rejuvenating influence of the sexual products. 



8. Material continuity of life. The actual continuity of living 

 matter is unbroken in consequence of the nature of cell division 

 and of the origin of the sexual products. We cannot, therefore, 

 yet conceive the origin of life, especially as all attempts to 

 demonstrate spontaneous generation have been unconvincing. 



9. Heredity. Every cell inherits the qualities of its parents, 

 though imperfectly. The resemblance of an animal to its parent 

 is due to the fact that a given cell of the parent cycle transmits 

 an influence to the child cycle, tending to cause a similar cell to 

 be developed in the same place and at the same time in the off- 

 spring. Heredity is imperfect, both inherently and from the 

 effects of external circumstances. 



10. Direct influence of external circumstances. This has now 

 become established in several cases. 



11. Predetermined union of cells. When the cells of one cycle 

 unite to form an animal, they arrange themselves definitely in 

 three sets (germ layers), at least in the higher metazoa. 



12. Vital union of cells. Some of the cells of each set are 

 united by means of the nerves into a common neural union or 

 association, each member of which loses some of its originality and 

 independence as an individual cell, and becomes able to affect the 



