CELL AGGREGATION AND DIVISION OF LABOR 93 



Animal Organizations. — The simplest orga,nization among animals is 

 the cell. Tlie parts of the cell perform different functions, but if sepa- 

 rated from each other cannot continue to exist. Many cells, such as 

 the Protozoa among animals and Protophyta among plants, exist as 

 separate individuals, not in contact with nor adhering to each other. 

 Examples of Protozoa of this sort have already been cited. Contrasted 

 with these are the loosely bound organizations among the Vorticellidse 

 in which any cell may leave the colony, seek out a suitable place, settle 

 down and secrete a new stalk, and by repeated divisions of itself and its 

 progeny produce a new colony. A closer organization exists in the aggre- 

 gations of the Volvox series already described. In Gonium, Pandorina, 

 and Eudorina the cells are of the same sort, performing the same functions. 

 It is possible that individual cells of these species might function and 

 perhaps reproduce the species if separated from the colony, but ordinarily 

 the mass of jelly holds them together. The cells of Pleodorina and Volvox 

 are less independent of the organization than are the cells of the other 

 members of the series. In these two genera the cells cannot function 

 properly after separation from their fellows, although they can exist for 

 a brief time. When the cells are separated reproduction is impossible. 

 In organizations of this character the cells are subordinated to the whole. 

 Differentiation of types of cells and division of labor have not pro- 

 gressed far, however, even in Volvox, the most complicated member of 

 this series. Its organization is relatively simple. More complex organ- 

 izations must be sought among the metazoa (many-celled animals). 



In the metazoan body the unit of structure is the cell, which is also the 

 functional unit. To refer to the analogy from industry, the cell of the 

 metazoan body is the individual worker. The cell is subordinate to the 

 organization, that is, to the organism as a whole. Beyond supplying its 

 own needs it performs but one function for the organization, or at most 

 a very few, and even its own requirements are in large measure met by its 

 fellows. It is no longer capable of performing all the general functions 

 of an independent cell like Paramecium or Amoeba. The needs of the 

 metazoan body are met by dividing up the functions among many kinds 

 of cells. All the cells of a kind performing the same function may occur 

 in a limited region and be organized into a tissue. The cells of this tissue 

 may secrete a digestive fluid and at the same time form the -lining of a 

 digestive cavity. Other tissues, each composed of its particular type of 

 cells which are suited to a given activity by reason of differentiation, 

 perform other functions, as contraction, the absorption of fluids, the 

 storage of fat, or the excretion of injurious substances from the circulat- 

 ing medium. Other tissues line cavities, others by virtue of their stiff 

 walls or lifeless material which they secrete give physical support to the 

 structure as a whole. Still another tissue receives stimuli and transmits 

 impulses in response to them. 



