194 C - M - CHILD. 



change in condition is differential with respect to the axis: the 

 apical region is most, the basal region least affected. When this 

 reduction of all regions to the same physiological level is complete, 

 the physiological gradient is obliterated and the fact that such 

 forms show no indications of polarity until new gradients are 

 determined in them is additional evidence for the conclusion that 

 the gradient constitutes the polarity. 



The fact that the basal region usually shows growth and 

 thickening far in excess of the normal may seem at first glance to 

 conflict with the interpretation of these modifications in terms of 

 differential inhibition, but, as a matter of fact such overgrowth is 

 a consequence of the differential inhibition. The basal region is 

 less inhibited than the apical, consequently it is more active 

 relatively to the apical region in the inhibited than in the normal 

 forms. This relatively greater activity enables it to obtain a 

 larger proportion of the available nutritive substance and so to 

 grow to a larger size, though more slowly than in normal develop- 

 ment. Up to a certain degree the excess and extension of basal 

 thickening increases with the decrease in apical, as compared 

 with basal activity. But with more extreme degrees of dif- 

 ferential inhibition all levels become more and more alike and 

 more basal as regards physiological condition and differences of 

 growth and development disappear, as in the spherical apolar 

 forms. The complete disappearance of these differences and the 

 reduction of the larva to a spherical apolar condition are more 

 likely to occur when the inhibiting factor has acted from the 

 early stages of development and has obliterated the gradient 

 before the differences in behavior of ectodermal and entodermal 

 regions have been established. 



As pointed out above, actual reversal of the physiological 

 gradient occurs in some cases, the reversal being indicated by- 

 reversal of orientation in locomotion and reversed susceptibility 

 and reduction gradients. In such cases the differential inhibiting 

 action of the agent is such that the original apical region is 

 reduced to a lower level of activity than the original basal region. 

 Since such reversal occurs only after basal thickening has begun, 

 it may result in a great increase of that thickening. The cases of 

 emigration of cells from the basal region (Fig. 6) suggest that the 

 polarity of these cells, i.e., the differences between the exterior 



