36 Elementary Biology. 



secretion, excretion, and respiration we have now to glance 

 at the general result of these changes on the protoplasmic 

 mass as a whole. 



Commencing at any given moment in the life of a cell, 

 we may find one of three things happening. *In the first 

 place, the chemical substances assimilated and undergoing 

 constructive metabolism may be just equal in amount to the 

 sum of the compounds used up in the concomitant destruc- 

 tive metabolism. In the second place, the amount assimi- 

 lated and absorbed may be greater than the amount 

 expended. Or thirdly, destructive metabolism may be in 

 excess of constructive metabolism. In the first case the 

 protoplasmic mass will be at a standstill it will neither 

 increase nor decrease in size ; in the second case it will 

 grow ; in the third case it will diminish in size and decay. 



Growth is accompanied usually by differentiation of parts 

 or development. Development may be either morpho- 

 logical or physiological or both, and consists in the gradual 

 adaptation of special parts to the performance of special 

 functions. 



Decay is generally ended more or less abruptly by death 

 or the cessation of the various manifestations of energy; no 

 doubt in consequence of a failure on the part of one or 

 other of the functions already mentioned, and the consequent 

 impossibility of maintaining a sufficient store of potential 

 energy on which to draw for the performance of the various 

 functions of the cell. As is to be expected, death is almost 

 always followed by a general breaking down of the complex 

 molecules through the unrestrained play ^f the laws of de- 

 composition. 



We have seen in the simple case which we have used as 

 an illustration that the various functions were all performed 

 by one cell. In other words, while we had physiological 

 differentiation we had complete, or almost complete, 1 



1 It is probable that the vacuoles may perform excretory functions 

 (P- 73)- 



