FROM ONE CELL TO MANY CELLS 75 



nuclei but no cell membranes around them a metazoon? If not, was 

 it a metazoon as soon as the cell membranes were formed? If not 

 then, was it a metazoon after some differentiation among those cells 

 had occurred? If the colonial theory is correct, was the first group of 

 adhering cells a metazoon or only a colony of single-celled animals? 

 Would a group of a thousand cells be a metazoon, while a group of 

 four was only a colony of protozoa? If number makes no difference, 

 would differentiation among the cells constitute the mark of a metazoon? 



Whatever the event that marks the advent of a metazoon, the 

 organism that has experienced that event is an individual. Without 

 that characteristic, it is a protozoon or a colony of protozoa, depending on 

 the nature of the origin of the metazoa. Biologists have differed in their 

 definition of the individual. To some, a group of cells that shows any 

 differentiation becomes a metazoan individual. Since in actual cases 

 when only one type of differentiation exists it is that between reproductive 

 and sterile cells, as in Pleodorina, defining the multicellular individual as 

 any group of cells in w^hich differentiation exists is equivalent to saying 

 that the individual is any group in which sterile cells are set apart from 

 reproductive cells. Other biologists have insisted that a group of cells is 

 not an individual unless its sterile cells are differentiated into several 

 kinds, as in Hydra. Under the former definition Pleodorina and Volvox 

 are individuals; under the latter they are colonies of unicellular organisms 

 exhibiting division of labor, since some reproduce and others do not. 



The distinction between reproductive and sterile cells is more funda- 

 mental than the distinctions among several kinds of sterile cells. In this 

 respect the former definition has the advantage. It is also preferable for 

 the reason that the criterion of individuality is, according to it, always the 

 same thing — loss of the capacity to reproduce by some of the adherent 

 cells — while under the latter definition the criterion of the individual 

 would presumably be a different distinction between sterile cells in every 

 line of descent. But definitions are arbitrary, and there is no tribunal 

 except usage which can choose among them. 



Further Organization. — Beyond the stage at which they are barely 

 "entitled to be called metazoa, most of the higher animals have gone long 

 distances. They have increased the number of their cells so that even 

 a moderate-sized animal contains literally billions of these units. With 

 increase in size, they have usually developed a framework or shell of some 

 sort wdiich provides protection or aids locomotion. Special devices are 

 created for the providing of food and the elimination of waste materials. 

 With large volume, they have had to provide means of communication by 

 which substances may be quickly transported from one part to another. 

 Structures of different sorts capable of effecting movement have arisen. 

 Unified control and the harmonious working together of the various parts 



