86 Cell Interactions during Growth and Morphogenesis 



remain in intimate association for long periods of time, in contrast to 

 the "Achlya" type of induction, which occurs when the reacting tissues 

 are separated by a considerable distance. Many chemical explanations can 

 be imagined to account for induction by direct contact but, as in much 

 of what we have already described, the necessary biochemical analyses 

 are only just beginning. 



Inhibitory Interactions 



As we stressed in Chapter 5, all morphogenetic fields involve inhibi- 

 tion— i.e., cells in one part of the field develop in a certain manner and 

 simultaneously inhibit the surrounding cells from doing so. This is the 

 case in hydranth development in coelenterates, bud development in plants, 

 the construction of the head, heart, limbs, and eyes in vertebrate embryos, 

 etc. As we mentioned in Chapter 5, inhibition can conceivably exist at 

 two levels: 



a. Where competition for a common store of materials in the en- 

 vironment enables only the cells that receive the greatest supply to de- 

 velop. 



b. Where the production of material by one cell inhibits another. 



Known Mechanisms of Cell Interaction 



Space limits us to only a few examples of known mechanisms by 

 which one cell can influence the growth and development of another. We 

 have drawn these from experiments with microorganisms which, being 

 simpler to manipulate and easier to grow under defined conditions, permit 

 more rigorous analyses at present. Unfortunately, these experiments in- 

 volve only single celled organisms and thus their interactions can serve 

 merely as models for similar interactions among the cells of higher plants 

 and animals. 



SELECTIVE INHIBITION DURING BACTERIAL GROWTH 



The bacterium Brucella abortus causes aborting fever in cattle and 

 Brucellosis in man. When B. abortus is taken from the blood of a diseased 

 animal and grown on an agar medium, it forms a colony with a very even 

 outline and mucoid texture, because each cell in the colony is surrounded 

 by a polysaccharide capsule. This type of colony is called "smooth," and 

 the constituent cells are called S-cells. During the growth of S-cells, rare 



