The materials, which we have accumulated in our laboratory In the past 

 10 years, indicate clearly that the resistance of various groups of fish to 

 poisons differs in different stages of ontogenesis. Periods of high stabil- 

 ity (eggs in the stage of pulsating heart, larvae in stage C2 and current 

 year fish) alternate with periods of low resistance (eggs in the stage of 

 gastrulation, fry in their early period, sexually immature individuals). 

 Particular attention should be given to the end of the larval period of 

 development and the beginning of the fry period, when toxicoresistance drops 

 sharply, approaching that of mature individuals, or falling somewhat below 

 it. On the whole, however, the resistance of various species to organic 

 poisons decreases with continuing ontogenetic development and reaches its 

 minimum in mature individuals. We relate this fact, observed repeatedly in 

 our laboratory, to the formation of various functional systems in the or- 

 ganism in ontogenesis and their neurohormonal control, which determines the 

 level of reactivity of the entire organism to various physical and chemical 

 irritants. An important role should be paid by the central nervous system 

 and its synaptic structures, since the toxic effects of many poisons in the 

 organic series are manifested by disruption of this activity, and conse- 

 quently dysfunction of the basic physiologic systems (Lukyanenko 1967). 



This point of view is held by a number of domestic researchers. In the 

 opinion of O.I. Shmalgauzen (1973), the younger stages of sturgeons (Caspian 

 sturgeon and Russian sturgeon) are more resistant to phenol than larvae as 

 they go over to active feeding. Whereas, a phenol concentration of 40 

 mg/liter is sublethal for eggs and only the teratogenic effect of phenol is 

 manifested, for larvae which have begun active feeding this concentration of 

 phenol is lethal. Larvae die with symptoms of acute phenol poisoning, des- 

 cribed for mature fish by O.I. Shmalgauzen (1973), indicating the "phenol 

 acts" on the larvae as a poison specifically damaging the nervous system 

 (page 7). 



An objective study of the resistance of various species of fish in early 

 ontogenesis to certain toxins was undertaken by A.F. Samylin (1974). Com- 

 paring the resistance of Salmo salar to ammonium carbonate during various 

 periods of ontogenesis, he came to the conclusion that as the eggs of the 

 fish increased, the survival time in the same concentrations of the sub- 

 stance decreased. A similar picture was observed in experiments with urea 

 (carbamide): Fry were less resistant to this toxin than eggs and larvae. 

 The decrease in resistance of salmon with increasing age observed in this 

 experiment was also seen in experiments with three pesticides; hexachlorane, 

 pentachlorophenol and copper naphthenate. We must note that the toxins used 

 in this work differ significantly in their mechanism of action and a number 

 of other properties, particularly their cumulative properties. Whereas am- 

 monium carbonate is a physiologically cumulative poison, hexachlorane is a 

 materially cumulative poison. Nevertheless, a decrease in toxicoresistance 

 with increasing age was observed in experiments with all of the substances. 

 Summarizing the results of the experiments, performed with five different 

 toxins, differing greatly in their degree of toxicity, the author emphasizes 

 that as ontogenetic development continues, the resistance of the salmon de- 

 creases. In complete agreement with our earlier published data on the age 

 dynamics of the resistance of fish to phenol (Lukyanenko 1967), A.F. Samylin 

 (1974), concludes that there is a significant change in the level of toxi- 



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