CHAPTER VIII. 



THE VARIATIONS IN STANDARD METABOLISM DURING THE LIFE 

 CYCLE OF THE INDIVIDUAL. 



WE have now to consider a group of phenomena to which in the pre- 

 sent state of our knowledge no definite cause of a physical or chemical 

 nature can be assigned, but which are nevertheless of the utmost im- 

 portance from a biological point of view. During the life of the in- 

 dividual, from the fertilization of the egg until the natural death of the 

 organism from old age, an immense number of morphological changes 

 take place, and these are regularly accompanied by corresponding 

 changes in the metabolic activity. We have to consider especially the 

 variations in metabolism taking place during development and growth, 

 during maturity and during senile decay. 



In the vertebrate animals and in most of the invertebrates we 

 have only one more or less continuous period of growth lasting some- 

 times until sexual maturity is reached, sometimes much longer, but in 

 the holometabolic insect we find intercalated between the larval and 

 the imago stages the pupal stage in which no growth in the ordinarily 

 accepted sense of that term takes place, but in which the morphological 

 structure is remodelled more or less completely. 



It will be convenient to deal first with the results obtained in ex- 

 periments on eggs and embryos of different animals, then with pupal 

 metabolism, and finally with the changes in metabolism taking place 

 during the remaining parts of the normal life cycle : postembryonic 

 growth, maturity and old age. 



Beyond these regular changes, which are more or less common to 

 all forms, we will also have to study in the present chapter certain 

 changes in metabolism which take place less regularly and in certain 

 forms only : the latent life of many invertebrates, the changes in meta- 

 bolism during breeding periods, etc., and the hibernation of mammals. 

 Our information with regard to these problems is certainly very im- 

 perfect, and will serve less to elucidate them than to indicate fields 

 which appear fruitful for future research. 



Metabolism During Embryonic Development. 



We have in the egg of an animal a single cell or perhaps rather 

 a single nucleus provided generally with an infinitesimal amount of 



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