CHAPTER V 



DIFFERENTIAL MODIFICATION OF DEVELOPMENT: 

 COELENTERATES AND FLATWORMS 



IN SO-CALLED ''normal" development only certain developmental 

 potentialities are realized; it is possible to alter the course of realiza- 

 tion experimentally by altering environmental conditions and so to 

 alter form and proportions of parts, localization of particular differentia- 

 tions, and even the general patterns of symmetry, asymmetry, and polar- 

 ity. We call the individuals thus produced "abnormal forms," "mon- 

 sters," "terata"; but it is scarcely necessary to point out that normal de- 

 velopment represents the reactions of the protoplasm concerned to a par- 

 ticular environment and abnormal development its reactions to other en- 

 vironments. 



Differential susceptibility was discussed in chapter iii, but certain 

 points concerning its relation to development are briefly recalled to atten- 

 tion here. Experiment has shown for many animal species that with ex- 

 posure in early developmental stages to an external inhibiting factor in 

 sufhciently high concentration or dosage to bring about more or less in- 

 hibition of all parts, but not rapidly lethal, the inhibition of development 

 is differential — ^in other words, a gradation in degree of inhibition, de- 

 creasing from the most to the least susceptible regions is evident. So far 

 as adequate data are available, the gradient pattern indicated by differ- 

 ential inhibition of development is, in general, essentially the same as that 

 indicated by other methods and is definitely related to the axiate pattern 

 of the organism (see chap. iv). 



With a certain lower range of concentration or dosage differential tol- 

 erance or acquirement of increased tolerance (a differential conditioning) 

 to many agents appears sooner or later following a primary inhibition. 

 Under these conditions differential modification of form and proportions 

 is opposite in direction to that in differential inhibition. The regions most 

 inhibited primarily show secondarily the greatest tolerance or condition- 

 ing. After temporary exposure to the inhibiting agent, provided its action 

 has not produced persistent injury, a differential recovery may occur, 

 with modification of development similar, in general, to that in differen- 

 tial tolerance and conditioning but often more extreme. If the more sus- 



i66 



