ORGANIC CHANGES 



71 



2. Cyclic Changes. — An equally familiar fact is that organisms 

 pass through a series of changes. The fertilised %g% undergoes 

 cleavage, the resulting cells grow and differentiate, an embryo 

 is formed, and gradually — often by circuitous paths — a minia- 

 ture form of the adult creature is attained. Out of apparent 

 simplicity an obvious complexity results. Growth still con- 

 tinues, often punctuated by resting periods, often rhythmic and 



Fig. 17. — Diagram of protoplasmic changes — B in plan, A in elevation. 

 The anabolic or constructive processes are represented to the left ; 

 the katabolic or destructive processes to the right. In B, one par- 

 ticular line of anabolic metabolism is supposed to be predominant. 



expressible in complex curves, often interrupted by peculiar 

 crises. Quickly or slowly the organism passes from youth 

 through adolescence to maturity, to its limit of growth and its 

 reproductive maturity. Quickly or slowly thereafter it sinks on 

 a down-grade towards death. As the old naturalists said, from 

 one period of vita minima the creature rises to a period of vita 

 maxima, and sinks back again into a vita minima which 



