FROM ONE CELL TO MANY CELLS 



67 



advantage — if advantage it be — of being capable of illustration by 

 organisms now living. The series of types used to illustrate it must 

 still show considerable gaps, and the representation is sure to be only 

 approximate; but the imagination can easily fill the vacant 

 places. Let us consider what these representative living 

 organisms may be. 



Types of Colonies. — The adherence of the two cells 

 produced by division should require no more explanation 

 than the physical connection and the mode of separation 

 seen in mitosis in multicellular animals. The fact that 

 protozoan cells should regularly separate is quite as remark- 

 able as that metazoan cells should regularly cling together. 

 Protozoan species in which the cells remain attached exist 

 in colonies. Sometimes no more than two cells adhere; 

 sometimes the number is thousands. The manner of 

 adherence varies. An envelope of jelly may help hold 

 the cells together, or they may be joined by stalks, or 

 the cells may cling to one another merely by small areas of 

 contact. 



Colonies take various forms. In Ceratium (Fig. 47), the cells are in 

 a single row, making what is called a linear colony. This type is rare in 

 animals but common in the simple plants (algae), in which cylindrical 

 cells are set end to end in long fine filaments. In some species the cells 

 do not touch one another but are joined by branching stalks (Fig. 48), 



Fig. 47.— 

 A linear 

 colony, Cera- 

 tium cande- 

 labrum. 



Fig. 48. Fig. 49. 



Fig. 48. — Codosiga cymosa Kent. A, treelike colony; B, individual cell in detail. 

 Fig. 49. — A gregaloid colony, Microgromia socialis. {From Calkins, "The Protozoa," 

 The Macm,illan Com-pany.) 



forming a treelike or dendritic colony. These branching colonies are of 

 many degrees of complexity, from those in which two cells fork off from a 

 single common stalk to ones in which the stalks branch and rebranch 

 and end in hundreds of cells. These organisms are all aquatic. The 

 branched stalks and cells may be quite exposed to the water, so that 

 currents of water pass freely among them, or they may be imbedded. 



