coresistance during various periods of ontogenesis. The least resistance 

 was noted in salmon in the stage of gastrulation in the embryonal period of 

 development, during transition of larvae to active feeding in the larval 

 period of development and during transformation of larvae to fry, i.e., in 

 the early fry period of development. These "points" of decreased resistance 

 of each of the three stages of early ontogenesis, found in experiments on 

 salmon with various toxins, are identical to those which we found in our ex- 

 periments (Volodin, et al_. 1966) with phenol using the eggs and larvae of 

 the zope and bream. 



Thus, at the present time there is sufficient proof of increased resist- 

 ance of the early stages of ontogenesis, primarily the embryonal period of 

 development, to organic poisons. The materials of a number of authors, in- 

 dicating that the resistance of fish in the early stages of ontogenesis to 

 organic poisons is significantly less than that of mature fish, are not in 

 agreement. For example, according to S.A. Patin (1977), developing eggs and 

 particularly larvae of the Stauridae are hundreds of times less resistant to 

 the effects of polychlorinated biphenols than are mature fish of related 

 species. He also noted higher resistance of embryonal and larval periods of 

 development in comparison to mature individuals in experiments with other 

 organic poisons, petroleum and surfactants. Recalling that these data do 

 not agree with many reports in the literature on the elevated resistance of 

 the embryonal period of life of fish obtained, true primarily with fresh- 

 water forms or transient forms, S.A. Patin assumes that one reason for the 

 disagreement is the salinity of the medium, which may change the toxic pro- 

 perties of detergents. We can add to this the fact noted earlier 

 (Lukyanenko 1967) of decreased resistance of sea fish in comparison to 

 fresh-water fish which, apparently, is true for all stages of individual 

 development. 



Still, it is difficult to understand the reasons for the reduced resist- 

 ance of the embryonal period of life in comparison to later stages of onto- 

 genesis to organic poisons. However, increased toxicoresistance in the 

 early stages of ontogenesis, in our opinion, is quite easily explained. As 

 we know, fish embryos in the early stages of development are protected by 

 the egg shell, which is an effective barrier for foreign substances, includ- 

 ing toxic substances (Skadovskiy 1955). This factor causes the unique con- 

 ditions of influence of organic toxins on the embryonal stage of development 

 of fish. No matter how toxic a substance dissolved in water may be, in 

 order to manifest its toxicity it must penetrate the egg shell and reach the 

 perivitelline fluid. The toxic effect is a function of concentration of the 

 substance and time of action. Therefore, it can manifest its action only if 

 a quantity of the substance accumulates in the egg sufficient to influence 

 the metabolic processes of the embryo and, in the final analysis, the course 

 of morphogenesis. It follows from this that the more difficult it is for a 

 substance to penetrate the egg shell, the less toxic it is for the embryo 

 still in the egg. Therefore, we must realize that in those cases when we 

 record increased resistance for the embryonal period of development of fish 

 to organic poisons, it is determined not only by the fact that the substance 

 has little influence on the metabolism of the developing organism, but also 

 the fact that the concentration of the substance penetrating through the 

 egg shell into the perivitelline fluid is significantly lower than that dis- 



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