14 HISTOLOGY 



grouping of cells to form a colony. In such colonization of cells we have 

 represented the first step toward multicellular organization. The rela- 

 tion of the cells to each other in these multicellular organisms varies with 

 the degree of advancement the colony has attained. This affords a 

 basis for dividing multicellular organisms into three orders of coloniza- 

 tion. The cells constituting any one of these kinds of colonies should 

 usually be considered as descendants of a single cell or otherwise closely 

 related. 



In a colony of the first order the component cells have no intimate, 

 vital relation with one another. This colonization results in a mere 

 increase in bulk, by which the cells are mutually protected against the 

 disturbing forces of their environment. Each cell of such a colony is 

 capable of living alone, if detached from the colony; and the colony 

 exists with an indefinite number of cells. In this case the cell and not the 

 colony must be considered the individual. Such colonies are found rep- 

 resented by certain Protophyta and Protozoa. 



In the second order of cell-colonies we find a more intimate relation 

 existing between the members of the cell aggregation. In a colony of 

 this type the number of component cells is always constant. The death 

 of one or more of the cells must result in their replacement by new cells. 

 In such a colony one cell cannot move independently of the others. This 

 is a comparatively simple mode of colonization. No cell is here con- 

 cerned in a peculiar manner with the life of the colony, each cell per- 

 forming all of the vital functions. Algae such as Pandorina, Eudorina, 

 and Gonium present this type of colonization. 



The third and highest order of colonization or cell aggregation presents 

 groups of cells vitally related to one another and in which the cells are not 

 all alike. A certain amount of differentiation has taken place, with the 

 result that certain cells with changed character are set apart to perform 

 definite functions. This is the type of cell-colony met with in all the 

 Metazoa. Phylogenetically this order of cell-colony probably would 

 fall under two divisions. One division would include the colonies in 

 which the differentiated cells remained independent of their fellows 

 which performed the same function. The reproductive cells, for exam- 

 ple, were not confined to a particular region of the colony, but were scat- 

 tered independently throughout the cell aggregate. An example of this 

 simple multicellular organism of the third order may be seen in Volvox 

 globator. In the second division of this third order of colonization we 

 meet with a higher grade of organization. In a cell aggregate of this 

 type the cells differentiated to perform a particular function are assembled 

 in a particular part of the body of the animal. The segregation of cells 

 similarly modified has given rise to what is known in histology as a tissue. 

 A tissue, then, is an aggregation of cells that have been specialized to 



