CHAPTER XVI 



PHASES IN THE LIFE OF THE INDIVIDUAL THE 

 DURATION OF LIFE AND THE CAUSE OF DEATH 



"TavTbv yap i7/3aW tivSpa KCU Trpe<r/3vv Oaveiv;" EURIPIDES, Alcestis. 



THE physiological life of the metazoon individual begins with the 

 union of sperm and ovum, and the organism thus formed thence- 

 forth proceeds to grow. As has been said by Verworn, 1 there 

 is an essential similarity between reproduction and growth, 

 both processes consisting of an increase of living substance. 

 " The difference between that which is usually termed growth 

 in the narrow sense and the phenomenon of reproduction consists 

 only in the fact that in the former case the newly-formed living 

 substance remains in constant connection with the original 

 organism and helps to increase its volume ; while in the latter 

 case a part of the substance separates itself from the original 

 organism, either, as in most cases, being set entirely free, or, as 

 in the increase of tissue-cells, being separated merely by a 

 partition-wall and remaining in place/' Among the more 

 highly organised Protozoa there are various transitional stages 

 between these two conditions. 



Growth, like reproduction, involves cell division. As the 

 mass of living substance increases, the cells must multiply, for 

 every cell has assigned to it a limit in size beyond which it 

 cannot pass. Cell division goes on, though with gradually 

 decreasing frequency throughout practically the whole of life ; 

 tissue formation continues, but from an early period of de- 

 velopment onwards there is a progressive diminution in the 

 power of growth. Increase in the number of cells is, how- 

 ever, specially characteristic of the embryonic period. In the 

 later stages of development growth occurs largely through cell 

 enlargement and the deposition of intercellular substance. 



1 Verworn, General Physiology, Lee's Translation, London, 1899. 



659 



