CHAPTER V. 

 THE LIFE CYCLE. 



Among the simplest organisms, in which each cell may 

 go on growing and dividing indefinitely, the familiar 

 phenomena of youth, maturity, and old age, are not 

 apparent. Every cell is a germ cell, and, therefore, ever 

 young. But with sexual reproduction comes in the dual 

 organism, composed of body plasm and germ plasm, only 

 the latter continuing, the former mortal. 



The normal life cycle. — It is the common lot, among all 

 organisms except the lowest, to be developed from an 

 egg, to be supplied in infancy with food, to pass through 

 hereditary changes of form, to grow and reach maturity 

 to exercise for a longer or shorter period the matured powers 

 of the body, to produce offspring, and then to grow old and 

 die. Youth, maturity, and age follow each other in an 

 orderly progression that is readily definable in terms of 

 metabolism, thus: 



1. In youth the building up processes are in the ascenden- 

 cy. Assimilation is greater than dissimilation (juvenescence). 



2 . In maturity the waste and repair of the body are on 

 a parity. Assimilation is equal to dissimilation. 



3. In age the building up processes are in a state of 

 relative decline. Assimilation is less than dissimilation 

 (senescence) . 



Although we may thus state in metabolic terms the his- 

 tory of the body, the explanation therefor must be stated in 

 terms of reproduction. It is necessary for the organism to 

 establish itself, before it can do much to provide for pos- 

 terity; hence, the nutritive apparatus of the body is 

 developed first, and growth precedes reproduction. The 



