Cell Interactions during Growth and Morphogenesis 81 



able duration before they can begin to reproduce actively, a lag that 

 results mainly from the fact that each cell must accumulate threshold 

 concentrations of key compounds (vitamins, amino acids, etc.) before 

 cell division can commence. Yet as fast as the cell produces these com- 

 pounds, they pour out into the medium by diffusion through the cell mem- 

 brane. As the external concentrations of these key substances rise, the 

 rate of diffusion out of the cell decreases, enabling the internal concentra- 

 tions to increase and finally reach the necessary thresholds. The time 

 needed to saturate the external environment with the critical substances 

 and so permit internal accumulation will depend on how fast the sub- 

 stances can be produced. Obviously, if each cell produces them at a 

 constant rate, two cells will produce twice as much per unit time as will 

 one and therefore do the job in half the time, i.e., many cells inoculated 

 into a given volume of fresh medium can begin growing very quickly, 

 whereas a few cells in the same medium would take a much longer time 

 or might never get started at all. 



Cooperative interactions also occur between embryonic cells. For 

 example, if we cut out a large piece of dorsal ectoderm from a chick or 

 mouse embryo and cultivate it in a dish with appropriate nutrients, the 

 tissue will yield nerve cells just as it would if left in the embryo. If, how- 

 ever, the single large piece is cut up into many small pieces and each is 

 cultivated separately, no nerve cells appear. The mere act of cutting does 

 not affect the result, because if the pieces are not separated but are cul- 

 tivated together in a compact mass, they produce nerve cells as efficiently 

 as before. Thus, many cells can do what few cannot, in this case become 

 transformed into specialized nerve cells. 



Inductive Interactions 



We have already dealt with one very fine example of induction— 

 namely, the production of male and female sex organs in the water mold, 

 Achlya (see Chapter 3). The properties of this system reflect perfectly 

 the general features of all inductive morphogeneses. 



1. The male sex organelle will not develop when the plant is grown 

 in isolation, but only when induced to do so by the presence of the female 

 plant. The development of the female sex organelle is similarly dependent 

 on the presence of the male (i.e., one cell group induces another to de- 

 velop along a specific path ) . 



2. The induced cells tnust be at the correct stage of development in 

 order to he receptive to the inductive signal. For example, after the female 

 plant has produced the large sacs, oospheres are induced to form in re- 



