ii6 PATTERNS AND PROBLEMS OF DEVELOPMENT 



physiologically old heads have a lower range of tolerance than those of 

 young animals and are sometimes killed by the primary toxic action of the 

 agent before they are able to acquire tolerance. If the heads do not die 

 during the first few days of exposure to the agent, they usually remain 

 alive indefinitely, or until starvation is far advanced, when the whole ani- 

 mal may die.^^ 



In KCN m/ioo,ooo most individuals live indefinitely; in KCN m 

 4/100,000 most or all die. Between these hmits, however, some degree of 

 differential tolerance appears in many individuals, with reversal of the 

 death gradient in the anterior zooid and acchmation of more or less of the 

 anterior region and of the posterior zooid region. Anterior ends of pieces 

 show greater tolerance or acchmation in regeneration than posterior ends 

 (Child, 1933c). In animals kept at 3°-5° C. a similar differential tolerance 

 appears in many individuals, with separation of the body into an anterior 

 piece and the posterior zooid by disintegration of the posterior part of the 

 anterior zooid, and later acclimation with regeneration of a posterior end in 

 the anterior piece and a head in the posterior piece at the same tempera- 

 ture. 



The point of chief interest in these experiments is the reversal in direc- 

 tion of the death gradient and the greater tolerance to low concentrations 

 of those regions which are most susceptible to high concentrations of the 

 same agents. They also constitute further evidence concerning the char- 

 acter of the longitudinal physiological pattern of the planarian and its 

 relation to reconstitution in pieces from different levels. The evidence at 

 hand from oxygen uptake, CO2 production, differential dye reduction, and 

 differential susceptibility to lethal effects of agents suggests that the more 

 tolerant regions, also more capable of acclimation, are more active regions, 

 regions of higher metabolism. In the case of alcohol, tolerance apparently 

 depends on abihty to oxidize it. In animals living in low concentrations of 

 alcohol for several weeks without feeding, the rate of oxygen uptake in- 

 creases greatly, sometimes 500 per cent, though the animals are sluggish 

 and do not lose weight more rapidly than controls in water (Buchanan, 

 1922, pp. 28, 29). In differential tolerance and acclimation the regions of 

 greater tolerance and acclimation are apparently regions of higher rate of 

 oxidative metabolism. As will appear in later chapters, evidence of differ- 

 ential tolerance and differential acchmation or conditioning within the in- 



'9 Child, 191 le, 1913a, 19146. It is perhaps unnecessary to note that in these experiments 

 on tolerance to alcohol the animals were kept in closed containers of large volume with small 

 air space, and solutions were renewed daily or every 2 days. 



