BIOLOGICAL INTRODUCTION 3 



first, by forming a colony of individuals; secondly, by instituting a 

 differentiation of cells, with a division of labour. 



In the first method, the formation of a colony of similar cells, 

 the cells, like the amo3ba, still carry out all the life-functions indi- 

 vidually ; each cell lives alone. The number of cells in such a colony 

 is not limited; it is but a group of individuals, not one individual. 

 Certain protozoa and protophyta exist as colonies. An example 

 is Carchesium (Fig. 3). 



In a little more advanced order of colonization the number of 

 members of the colony is limited and kept constant, death of a member 

 being followed by replacement. Such a colony is generally ensheathed 

 by a wall, so that the cells have no independent movement. With 

 these limitations, each cell of the colony performs its vital functions 

 independently. An example of such a colony is Gonium (Fig. 4). 



FIG. 3. A COLONY OF INDIVIDUALS OF CARCHESIUM, ATTACHED BY A COMMON BRANCH- 

 ING STALK, SHOWING /, THE CONTRACTILE STALK OF ONE INDIVIDUAL. (Remy, 

 redrawn after Dahlgren and Kepner.) 



In the second method a differentiation of cells, with a division of 

 labour, takes place. Here the colony becomes the individual not a 

 group of individuals. The members of this group range from a 

 simple multicellular organism to man. Volvox globator (Fig. 5) 

 represents the beginning of this type of organization. In it certain 

 cells become differentiated for the main function of life reproduction 

 each of the other cells performing all the other functions. 



In the higher organizations certain cells become segregated and 

 form a tissue that is, a group of cells performing more or less one 

 definite function. Later the differentiated tissues become grouped 

 into organs, each having some particular function. Nevertheless, 

 each cell remains a separate living unit, having its own share of work 

 to perform. 



The developmental history ontogeny of the multicellular organ- 



