46 The Development of a Primi-Uve Animal 



and by the number of head cells in area II. The complete description of 

 the rate of head development in area I would then become: 



rate of head /developmental \ /number of \ /autosensi-\ 



development = I potential j X I stem cells j — I tivity in J 



in area I \ in area I / \ in area I / \ area I / 



(number of\ /heterosen-X /number of \ 

 head cells j — I sitivity J X I head cells J 

 in area I / \ in area I / \ in area II / 



The same sort of equation can be written for the rate of head development 

 in area II. 



We can now ask how area I can gain dominance over area II and 

 prevent it from forming a head, and we see that this can be done in a 

 number of ways : 



1. Area I could have a much higher developmental potential or a 

 much greater number of cells. In either case, it would form a head so fast 

 and begin pouring out inhibitory materials so soon that area II wouldn't 

 have a chance to get started. 



2. Area I might be very resistant to auto-inhibition (i.e., the inhibi- 

 tory materials might be carried away by the body fluid circulation quickly, 

 whereas area II might be very sensitive (i.e., the inhibitory materials 

 might not be carried away ) . 



3. Area I might not be very heterosensitive, while area II might be 

 extremely so. 



4. Even were developmental potentials, sensitivities, and cell num- 

 bers equal, area I could gain dominance by starting head formation first. 

 The head start (forgive me) would permit it to inhibit area II. 



Any one of these conditions, or a combination of them, would serve to 

 establish the dominance of area I. 



At present, none of the biochemical and genetic mechanisms that 

 create differences in developmental potential or that exert inhibitory ef- 

 fects have yet been identified for any morphogenetic field. Thus, this kind 

 of developmental study is in its infancy. 



