Cell Interactions during Growth and Morphogenesis 91 



convert both citrulline or ornithine to the needed final product, it grows 

 very well if it is streaked next to either mutant 2 or mutant 3. In multi- 

 cellular organisms, the various tissues may well influence the growth of 

 neighboring cells in just this manner. There are numerous examples in 

 embryonic development of the effect of one organ or tissue upon the 

 growth of another. 



Conclusion 



All cells in nature interact with one another. Sister microbial cells 

 compete with each other for food and space, and sometimes exert direct 

 inhibitory effects on each other in order to survive the competition. They 

 can also assist each other by exchange of metabolites. Genetic information 

 can be transferred from one to another and produce new capacities in the 

 recipients. In multicellular forms, where the constituent cells are packed 

 together in a compact mass, opportunities for cell interaction are very 

 great and, since developmental activities must be timed perfectly, are 

 even more necessary than in single-celled organisms. The kinds of inter- 

 action can be classed as follows: 



a. population-density effects— where many cells of the same kind co- 

 operate to do something that few together could not do. 



b. inductive interactions. 



c. synergistic inductions. 



d. inhibitory interactions. 



The vehicles of interaction seem to fall into these categories: 



a. Interactions promoted by diffusible agents between cells sepa- 

 rated from one another. Such agents can be carried by simple diffusion or 

 transported through the circulatory system. 



b. Direct-contact interactions between the effector and effectee cells. 

 Such systems probably require intimate contact because the material 

 transferred is very unstable and will not survive transport over great dis- 

 tances or because large amounts of cytoplasm are exchanged and several 

 substances must travel as a unit. 



c. Contact interactions without exchange of material. The surface of 

 one cell reacts with that of another to create drastic alterations of the sur- 

 face, which are then reflected by secondary changes in the metabolism of 

 the cell. 



