CHAPTER XVI 



PHASES IN THE LIFE OF THE INDIVIDUAL THE 

 DURATION OF LIFE AND THE CAUSE OF 

 DEATH. 



" Tavrbv yap r}/2aW avSpa Kal Trpe(r/3vv daveiv " EURIPIDES, Ahestis. 



THE physiological life of the metazoon individual begins with the 

 union of sperm and ovum, and the organism thus formed thenceforth 

 proceeds to grow. As has been said by Yerworn, 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 development onwards there is a progressive 

 diminution in the power of growth.' Increase in the number of cells 

 is, however, specially characteristic of the embryonic period. In the 

 later stages of development growth occurs in great measure through 

 cell enlargement and the deposition of intercellular substance. 



Minot has compared the growth of the body to a man building 

 a wall. 2 " He begins at first with great energy, full of vigour ; the 

 wall goes up rapidly ; and as the labour continues, fatigue comes into 



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



2 Minot, " The Problem of Age, Growth, and Death," Popular 8cin<*e Monthly, 

 vol. Ixxi., 1907 ; reprinted London, 1908. 



