176 PATTERNS AND PROBLEMS OF DEVELOPMENT 



from the regenerated tissue, as normally. In these cases the anterior re- 

 generating tissue of the piece apparently undergoes conditioning more 

 rapidly, or to a greater degree, than the posterior end. These secondary 

 modifications have been obtained with KCN, ethyl alcohol, chloretone 

 and other anesthetics, CO^, hydrogen ion, and low temperature. With 

 cyanide they develop very slowly, usually becoming evident only after sev- 

 eral weeks' exposure; in alcohol they begin to appear in a few days; with 

 other agents used, they develop at different rates between these extremes. 

 The history of these pieces suggests that immediately after section 

 there is little difference in condition at the two cut ends, but later there 

 is evidently a difference. If the pieces remain in water 48 hours after 

 section and are then subjected to the inhibiting agent, anterior regenera- 



A B C 



Fig. 62, A-D. — Anteroposterior differential tolerance and differential conditioning to in- 

 hibiting agents in pieces of Dugesia dorotocephala. A and B, early, C and D, later, stages of 

 differential tolerance or conditioning, showing greater inhibition of posterior reconstitution. 



tion is more inhibited than posterior; that is, a direct differential inhibi- 

 tion occurs. This is more clearly evident with agents such as cyanide, in 

 which the secondary modification occurs very slowly. Measurements of 

 length and width of anterior and posterior new tissue make it possible to 

 compare areas of anterior and posterior regenerating tissue in controls 

 and experimental pieces; since thickness of the two regions is much the 

 same, the measurements serve as a rough ratio of amounts of anterior 

 and posterior regenerated tissue. The following data serve as examples: 



Anterior regeneration 

 Posterior regeneration 



Controls, 8 days in water i -43* 



. f 2 days in water, 6 days in KCN m/ 100,000. . i.ii 



\ Same lot after 14 days in KCN m/ioo,ooo . . i . 66 



• The figures are the sums of products of lengths and widths of anterior, divided 

 by the sums of products of lengths and widths of posterior regenerated tissue, as 

 determined in each piece of the experimental lot. 



In the experimental lot the ratio of anterior to posterior regeneration 

 after 6 days in cyanide is far lower than in the control, that is, inhibition 



