414 C. M. CHILD 



parently specific relations between particular organs or tissues 

 and particular agents. Many different lines of evidence indi- 

 cate that the physiological axes are primarily quantitative 

 gradients in physiological condition and metabolic rate and that 

 the differential susceptibility at different levels of an axis re- 

 sults from this difference (Child, '20 b). The region from 

 which the head, or apical end develops, being primarily the 

 most active region, is most susceptible to higher and most capable 

 of acclimation to lower concentrations and intensities of external 

 agents. It has been demonstrated in various ways that this 

 relation holds good for the head of Planaria (Child, 'lie, '12, 

 '13 a, b, '16 a). 



It has also been shown that differential susceptibility, be- 

 sides determining different survival times of different regions, 

 determines developmental modifications in two opposite direc- 

 tions, differential inhibition on the one hand and differential 

 acclimation or recovery and in some cases differential accelera- 

 tion on the other (Child, '16 b, '17; Bellamy, '19). The graded 

 series of head-forms in Planaria is evidently the result of a non- 

 specific action, since all terms of the series are produced by 

 many different external and internal conditions. Moreover, 

 since different regions of the head are affected in different degree 

 in the different forms, a differential action of the determining 

 factor or factors is evidently concerned. It remains, then, to be 

 determined whether, or to what extent, the series of head-forms 

 can be interpreted in terms of differential susceptibility. 



DIFFERENTIAL INHIBITION IN HEAD-DEVELOPMENT 



The normal head. The normal head of Planaria dorotocephala 

 (fig. 1) possesses certain rather definite characteristics as regards 

 external form localization of sense organs and position, form 

 and structure of the cephalic ganglia. The ganglionic mass, un- 

 questionably the primary organ of the head, is in the normal 

 head, distinctly double, consisting of right and left halves, 

 connected by a commissure. The same bilaterality is also 

 evident in the localization and innervation of the sense organs 



