6 DIVISION OF LABOUR. 



to a physiological unit, corresponding to but greatly more complex 

 than the chemical molecule. This unit to remain a physiological 

 unit and to continue to live must contain not only a portion of 

 the living substance but also the food for that living substance, 

 in several at least of the stages, from the initial raw food up to the 

 final ' living ' stages, and must similarly contain various stages of 

 waste. 



6. Now the great characteristic of the typically simple 

 amoeba (leaving out the nucleus) is that, so far as we can ascer- 

 tain, all the physiological units are alike ; they all do the same 

 things. Each and every part of the body receives food more or 

 less raw and builds it up into its own living substance; each 

 and every part of the body may be at one time quiescent and 

 at another in motion; each and every part is sensitive and 

 responds by movement or otherwise to various changes in its sur- 

 roundings. 



The body of man, in its first stage, while it is. as yet an ovum, 

 if we leave aside the nucleus and neglect differences caused by the 

 unequal distribution of food material or yolk, may also be said to 

 be composed of like parts or like physiological units. 



By the act of segmentation however the ovum is divided into 

 parts or cells which early shew differences from each other ; and 

 these differences rapidly increase as development proceeds. Some 

 cells put on certain characters and others other characters ; that 

 is to say, the cells undergo Jiistological differentiation. And this 

 takes place in such a way that a number of cells lying together 

 in a group become eventually converted into a tissue ; and the 

 whole body becomes a collection of such tissues arranged together 

 according to morphological laws, each tissue having a definite 

 structure, its cellular nature being sometimes preserved, sometimes 

 obscured or even lost. 



This histological differentiation is accompanied by a physio- 

 logical division of labour. Each tissue may be supposed to be 

 composed of physiological units, the units of the same tissue being 

 alike but differing from the units of other tissues ; and corre- 

 sponding to this difference of structure, the units of different 

 tissues behave or act differently. Instead of all the units as in 

 the amoeba doing the same things equally well, the units of one 

 tissue are told off as it were to do one thing especially well, or 

 especially fully, and thus the whole labour of the body is divided 

 among the several tissues. 



7. The several tissues may thus be classified according to 

 the work which they have to do ; and the first great distinction is 

 into (1) the tissues which are concerned in the setting free of 

 energy in special ways, and (2) the tissues which are concerned 

 in replenishing the substance and so renewing the energy of the 

 body. 



Each physiological unit of the amoeba while it is engaged in 



