PHYSIOLOGICAL GRADIENTS. 159 



relations as regards median, lateral and dorsal were not entirely 

 clear. As regards the alimentary tract, various facts indicate 

 that in the adult the region of greatest activity and highest sus- 

 ceptibility is in the middle of the body about the base of the 

 pharynx and that a gradient of decreasing activity extends in 

 both directions from this region, but since the alimentary tract is 

 an internal organ and is not readily separable from other parts, 

 it is difficult to obtain conclusive evidence on this point. The 

 antero-posterior gradient of ectoderm and body-wall persists 

 throughout life, but with the appearance of new zooids at the 

 posterior end new gradients arise in that region, or more strictly 

 speaking, the original gradient undergoes modification. Ap- 

 parently the primary embryonic relations have undergone more 

 or less alteration, even in Planaria. 



With the appearance of more or less specific relations between 

 particular regions or organs and particular agents, the value of 

 the susceptibility method as a means of distinguishing general 

 quantitative differences and relations is of course greatly de- 

 creased. In the higher animals apparent specificity of relation 

 between particular organs and particular agents is much more 

 evident than in lower forms and becomes increasingly complex 

 with the progress of differentiation, but even in these forms the 

 general non-specific susceptibility relations appear in the earlier 

 developmental stages, at least in all forms examined. While 

 caution is always necessary in interpreting the data of suscep- 

 tibility in non-specific quantitative terms, it has become more and 

 more evident as the data have accumulated that general non- 

 specific susceptibility relations do exist, particularly in the sim- 

 pler organisms and in the earlier stages of development, and 

 that they are indications of fundamental physiological features 

 of organismic pattern. 



It has also been possible to control and modify development in 

 definite predictable ways through the differential susceptibility of 

 different levels of the axial gradients. Such modifications con- 

 sist in differential inhibition, differential acceleration, differential 

 acclimation and differential recovery, each representing a definite 

 teratological type. In cases of differential inhibition with re- 

 spect to a gradient the degree of inhibition varies directly with 



