426 C. M. CHILD 



figures 29 to 33, while in recovery after removal of the inhibiting 

 factor the more nearly normal head-forms may appear. In 

 temperature experiments, for example, pieces which remain 

 acephalic for several weeks at low temperature (3° to 5°C.) 

 sometimes begin to develop anophthalmic or teratomorphic 

 heads after this time, even when kept at the low temperature, 

 but if removed to 20°C. many develop teratophthalmic and 

 some even normal heads. 



DIFFERENTIAL SUSCEPTIBILITY IN DEVELOPMENT IN GENERAL 



It will be noted that in the heads of Planaria, as well as in 

 the development of the sea-urchin (Child '16 b), the polychete 

 (Child, '17) and the frog (Bellamy, '19) departures from the 

 normal occur under the experimental conditions employed in two 

 opposite directions. In the differential inhibitions certain regions 

 are affected to a greater degree than others, and the parts to which 

 the regions most affected normally give rise are more or less 

 reduced or completely absent. In the differential acclimations 

 and recoveries the parts originally most inhibited develop more 

 or less completely or may even show over-development. The 

 most extreme types of differential acclimation (figs. 19, 20, 23) 

 have thus far been observed only after the use of external agents 

 which produce at first marked inhibition, but in which a con- 

 siderable degree of acclimation occurs. The habit-forming 

 inhibiting drugs, for example, so far as they have been used, are 

 particularly favorable agents for the appearance of these second- 

 ary changes. In cyanide, on the other hand, in which acclimation 

 is very slight and occurs very slowly, the more extreme acclima- 

 tion forms have never been seen. All other head-forms except 

 these extreme acclimation forms, not only the differential inhibi- 

 tion series normal to acephalic, but the less extreme acclima- 

 tion forms, can be and have been determined by all of the external 

 chemical and physical agents used in my experiments, as well . as 

 by the physiological conditions associated with differences in 

 size of piece, region of body, nutrition, age, etc. These head- 

 forms are, in short, the morphological effects of physiological 



