CELL AGGREGATION AND DIVISION OF LABOR 91 



L)ififerentiation. — Modification from the simple to the complex, like 

 that which has apparently occurred in the development of Volvox, 

 may be called differentiation, which means a becoming different. This 

 term is also used concretely to denote the end result of the change. Dif- 

 ferentiation always involves structural changes of some sort, such as 

 changes in size, form or proportions, and also internal changes, some of 

 which may be invisible. Differentiation, moreover, always involves 

 specialization, and is always accompanied by division of labor. By special- 

 ization is meant limitation of function to one or a few processes; by 

 division of labor, the parceling out of various functions among cells, 

 structures, or individuals. The most obvious applications of the terms 

 differentiation and division of labor are found in complex animals, but 

 they may be properly employed for very simple cells. For instance, 

 simple cells are not made up of a homogeneous material. Their pro- 

 toplasm is differentiated into parts, as was pointed out in Chapter II. 

 In Amoeba there are relatively few parts, in Paramecium there are many 

 more parts. In other words, differentiation has gone farther in the 

 latter animal than in the former. 



It is not sufficient, however, to regard differentiation as referring 

 entirely to structural features since differentiation also involves func- 

 tion. Each differentiation of protoplasm as manifested in some structure 

 of a cell is concerned with the performance of a certain function or at 

 most with a limited group of functions. In Paramecium, for example, 

 a certain part of the cell is concerned with locomotion, another with 

 the expulsion of waste materials, another with movements incident to 

 food-taking. Other portions secrete digestive enzymes. The micro- 

 nucleus is concerned with sexual reproduction and perhaps with certain 

 other functions. Thus even in single cells there is division of labor. 



Division of labor is more apparent, but no more real, in some cell 

 aggregations than in the single cell. In Volvox certain cells (the somatic 

 cells) maintain the form of the body and care for the general functions 

 of the entire group, such as locomotion, nutrition, and responses to stimuli ; 

 cells of another type (the parthenogonidia) are set apart for the multi- 

 phcation of the species by asexual methods; and still other cells (the 

 true germ cells) for the sexual method of reproduction. The somatic 

 cells are incapable of functioning as germ cells. Each type of cell has 

 a definite duty or circumscribed group of duties to perform. In a word 

 each cell is a specialist in its work. Its structure is such that it can per- 

 form its function sufficiently well for the species to maintain itself and 

 when each cell performs its functions properly all the needs of the aggre- 

 gation are met. Division of labor is thus a type of cotipcration accompany- 

 ing differentiation in whicih the sum of the functions of the parts makes ujj 

 the functions of the organism as a whole. 



