194 PATTERNS AND PROBLEMS OF DEVELOPMENT 



The shorter the piece at a given body-level and the farther posterior 

 in the anterior zooid the level of origin, the more effective is the physio- 

 logical inhibiting factor. The length of piece at which inhibition of the 

 head begins to appear increases from anterior to posterior levels of the 

 anterior zooid. At the anterior level the physiological factor is effective 

 only in very short pieces. The intensity or rate of activation of the head- 

 forming cells and the rate of head development decrease from anterior 

 to posterior levels (p. 43), and it may be that the nervous effect of 

 posterior section is better transmitted in the anterior direction at more 

 posterior levels. Either or both of these conditions may determine the 

 greater effectiveness at a given distance and effectiveness at a greater 

 distance of the physiological inhibiting factor at more posterior levels. 



In any lot of similar pieces head frequency depends on the relation be- 

 tween the condition of the cells directly concerned in head formation and 

 the physiological factor preventing those cells from undergoing the change 

 in condition necessary for complete head development. The physiological 

 factor has been called an "inhibiting factor" because it inhibits head de- 

 velopment differentially, but it appears actually to be a factor which tends 

 to maintain the cells as functional parts of a certain region of the body. 

 In order to give rise to a head, the cells must become free from functional 

 relation to other parts of the body; and the physiological differential in- 

 hibition of head development represents all degrees of inability to attain 

 that freedom, because the nervous stimulation resulting from posterior 

 section tends to maintain another sort of cellular behavior. The parts of 

 the head which require the highest level of activation for initiation of their 

 development are first and most inhibited; these are the median head 

 regions. With lesser degrees of freedom more lateral regions are reduced 

 or prevented from developing, until, when the cells are completely domi- 

 nated by the physiological effect of posterior section, they do not react 

 at all to the absence of regions anterior to their level, and acephalic forms 

 result. According to length of piece, level of origin, physiological condition 

 of parent animal, and nature and concentration of external agent and 

 period of exposure, the degree of differential inhibition of head develop- 

 ment varies in definite, orderly ways. Experimental control of head fre- 

 quency is possible by controlling length of piece and level of origin and 

 condition of parent animal and also by external inhibiting and accelerat- 

 ing agents. All these methods of control are merely ways of controlling 

 and altering the balance between the two antagonistic factors concerned 

 in determining the degree of differential inhibition of head development. 



