114 PATTERNS AND PROBLEMS OF DEVELOPMENT 



the general gradient pattern, seems evident. They are also of interest be- 

 cause degree of inhibition of head regeneration in pieces parallels the 

 increases in susceptibility following section ; and if the increases are pre- 

 vented, head development is not inhibited (see pp. 177, 406). The changes 

 in rate of oxygen uptake found by Hyman are, so far as determined, es- 

 sentially parallel to the changes in susceptibility. 



In low concentrations which kill very slowly but are still above the lim- 

 it of tolerance (e.g., KCN m/ 10,000), more than one posterior zooid, if 

 present, may become distinguishable by slight differences in susceptibility 

 (Fig. ^T), H, I, J). As regards the death gradient of the digestive tract, 

 it is difficult to attain certainty, for direct exposure, at least to chemical 

 agents, depends more or less on disintegration of the body wall. There is, 

 however, some evidence that in well-fed animals death progresses from the 

 pharyngeal region anteriorly and posteriorly. In agents which penetrate 

 readily, such as cyanide, the gut of well-fed animals disintegrates as early 

 as, or earlier than, the body wall; but in starved animals it is much less 

 susceptible and may still be intact after the body wall has disintegrated. 



In the earlier stages of reconstitution the new tissue at both ends is 

 more susceptible than the old, and the developing head more susceptible 

 than the posterior tissue; in the old tissue susceptibility decreases from 

 the head posteriorly. When a head develops at the posterior end of a 

 piece or in any other than the usual position, a longer or shorter suscepti- 

 bility gradient arises in relation to it. 



Attention must be called to the fact that the anteroposterior gradient 

 pattern in postembryonic stages of the planarian body does not represent 

 a gradient of growth in body length. The rate of growth increases poste- 

 riorly, as is at once evident from comparison of individuals of different 

 length; Abeloos (1928) has given statistical data concerning this point. 

 The physiological gradients are not necessarily growth gradients and may 

 or may not parallel such gradients (see Needham, 1931, p. 584). 



Evidence of differential tolerance and differential acclimation or con- 

 ditioning has been obtained with D. dorotocephala . In low concentra- 

 tions of ethyl alcohol (1-1.5 P^'" cent) most individuals remain alive and 

 intact for at least a week or two and show more or less evidence of in- 

 creased tolerance or acclimation. During the first few days they are more 

 or less anesthetized, but motor activity increases after several days of ex- 

 posure. Small young individuals, whether from eggs or reconstituted 

 pieces, while more susceptible than large individuals to higher concentra- 

 tions of alcohol and at first less active than large in these low concentra- 



