INTRODUCTION. 21 



lodged, or with which are built up in some way or other, food and waste in 

 various stages. 



Now, an amoeba may divide itself into two, each half exhibiting all the 

 phenomena of the whole ; and we can easily imagine the process to be 

 repeated, until the amoeba was divided into a multitude of exceedingly 

 minute amoebae, each having all the properties of the original. But it is 

 obvious, as in the like division of a mass of a chemical substance, that the 

 division could not be repeated indefinitely. Just as in division of the chem- 

 ical mass we come to the chemical molecule, further division of which 

 changes the properties of the substance, so in the continued division of the 

 amoeba we should come to a stage in which further division interfered with 

 the physiological actions, we should come to a physiological unit correspond- 

 ing 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 typical amoeba (leaving out the 

 nucleus) is that, as far as we can ascertain, 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 surroundings. 



The body of man, in its first stage, while it is 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 show differences from each other ; and these differences 

 rapidly increase as development proceeds. Some cells put on certain char- 

 acters and others other characters that is to say, the cells undergo histologi- 

 cal 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 physiological division 

 of labor. 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 corresponding 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 

 labor 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 renew- 

 ing the energy of the body. 



Each physiological unit of the amoeba while it is engaged in setting free 

 energy so as to move itself, and by reason of its sensitiveness so directing 

 that energy as to produce a movement suitable to the conditions of its sur- 

 roundings, has at the same time to bear the labor of taking in raw food, of 

 selecting that part of the raw food which is useful and rejecting that which 



