CHAPTER XIX 



THE LIFE CYCLE 



231. Birth, growth and development, and death. Certain 

 phenomena are familiar to us as occurring inevitably in the 

 life of every animal. Each individual is born in an imma- 

 ture or young condition ; it grows (that is, it increases in 

 size), and develops (that is, changes more or less in struc- 

 ture), and dies. These phenomena occur in the succession 

 of birth, growth and development, and death. But before 

 any animal appears to us as an independent individual 

 that is, outside the body of the mother and outside of an 

 egg (i. e., before birth or hatching, as we are accustomed to 

 call such appearance) it has already undergone a longer 

 or shorter period of life. It has been a new living organ- 

 ism hours or days or months, perhaps, before its appear- 

 ance to us. This period of life has been passed inside an 

 egg, or as an egg or in the egg stage, as it is variously 

 termed. The life of an animal as a distinct organism be- 

 gins in an egg. And the true life cycle of an organism is 

 its life from egg through birth, growth and development, 

 and maturity to the time it produces new organisms in 

 the condition of eggs. The life cycle is from egg to egg. 

 Birth and growth, two of the phenomena readily apparent 

 to us in the life of every animal, are two phenomena in the 

 true life cycle. Death is a third inevitable phenomenon in 

 the life of each individual, but it is not a part of the cycle. 

 It is something outside. 



232. Life cycle of simplest animals. The simplest ani- 

 mals have no true egg stage, nor perhaps have they any true 



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