CHAPTER V 



CELL AGGREGATION, DIFFERENTIATION, AND DIVISION 



OF LABOR 



Even a brief survey of living things reveals the fact that there is a 

 great diversity in size and complexity of animals. At one extreme are 

 animals so small as to require the aid of the microscope to see them and 

 so simple in structure as to cause wonder that they are able to maintain 

 their life processes, while at the other extreme are animals of great com- 

 plexity and sometimes of enormous size. Examples of the first group 

 are the Protozoa (one-celled animals); while insects, fish, birds, snakes, 

 and such mammals as the mouse and the whale illustrate the second 

 group. Between the extremes of simplicity and complexity many 

 gradations occur, so that animals may be arranged in an ascending or 

 descending scale of organization. In such a scale it is found that in 

 certain species the individual consists of a single cell and that in species 

 not widely differing from them the cells are grouped into some form of 

 aggregation. Furthermore, some aggregations exhibit a slightly more 

 complicated relationship of the cells to one another than do other 

 aggregations in which the cells are very similar. That is, the scale at 

 many places is a finely graded one, and the nature of the steps from one 

 member of the series to the next may be various. These differences in the 

 method of aggregation open wide possibilities for differences in physiolog- 

 ical influence or dependence of cells upon each other. They also raise 

 important questions regarding the origin and significance of the higher 

 degrees of complexity. Changes in structure and changes of function 

 of the cells or individuals during the evolutionary development of the 

 complicated forms are strongly suggested, and speculation upon the 

 nature of these changes is invited. Almost the only source of information 

 regarding these questions is an examination of the different types of 

 aggregation now found in animals and the probable relationships that 

 exist among them. 



From Single Cell to Colony. — The point of departure in socking the 

 facts in regard to cell aggregation is obviously the single cell as it exists 

 in the majority of protozoan species. In these species the cell carries on 

 the functions of the body. It takes in food, digests it, and assimilates 

 the products of digestion. Within this one cell are carried on the proc- 

 esses of respiration, secretion, excretion, and locomotion. It responds 

 to stimuli, and it produces other cells. Thus the cell in such protozoan 

 species carries on all its life processes independently of its fellows. Its 



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