The Development of o Primitive Animal 45 



since, if the two areas were isolated from each other, each would con- 

 struct a head at its characteristic rate and neither would dominate the 

 other. It is necessary for the two areas to be able to interact upon one 

 another. 



Suppose the developing head cells can pour out materials that inhibit 

 others from becoming head cells. First of all, the cells in area I that had 

 already been incorporated into the developing head would inhibit the 

 remaining stem cells in that area from changing into head cells. The 

 degree of inhibition would depend on the inherent sensitivity of the area I 

 stem cells toward being inhibited by their own head ( autosensitivity ) and 

 on the number of head cells that are producing the inhibitory materials. 

 This inhibition would have to be subtracted from the rate of head de- 

 velopment defined in statement 1. Thus, 



rate of head /developmentalX /number of \ 

 development = ( potential ) X ( stem cells J 



;a I / 



in area I \ of area I / \ 



(auto- \ /number of\ 

 sensitivity | X ( head cells J 

 in area I / \ in area I / 



According to this statement, the rate of development would be very high 

 at the beginning of head formation. Then, as the head grew larger and 

 more and more cells were incorporated into it, they would produce in- 

 hibitory materials and detract materially from the rate at which additional 

 stem cells would be incorporated into the head. Meanwhile, the number of 

 stem cells would decrease as they were converted into head, and the first 

 term on the right side of the above equation would decrease. Eventually, 

 the two terms on the right side would cancel out, and the rate of head 

 development would become zero (i.e., the head would now be complete). 

 In the case under consideration, the rate would fall to zero before all the 

 stem cells had been converted into head cells, since a coelenterate head 

 would look pretty silly without a stem to support it. 



Now suppose the two areas are in contact with each other ( for ex- 

 ample, if the two have a common coelenteron and are bathed by the same 

 body fluid). Then the head cells of area II would inhibit the stem cells 

 in area I in the same fashion as described above. That is, the rate of area I 

 development will be impeded by the inherent sensitivity of area I stem 

 cells to the inhibitory products ^ of area II head cells ( heterosensitivity ) 



1 1 have described the interaction between areas I and II as the result of the 

 production of inhibitory materials. We could equally well account for it by assuming 

 that the areas compete for a common pool of nutritive materials that serve to stimulate 

 development. Were this the case, area II would inhibit area I by depriving it of these 

 materials and vice versa. Actually, the mathematical argument turns out to be the 

 same whether we put it in terms of inhibition or deprivation. 



