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were still alive in these bowls; it thus seems as if the tadpoles die 
sooner and in greater number, while being in higher concentration. 
This result does not agree with Huirscn’s’*) results, who supposes 
that the quickest development takes place in the concentration, 
nearest to the concentration in which life is impossible. In this case 
one should expect a quicker development in the higher concentrations 
than in the lower-ones, but this has not been proved. 
Moreover Hirsca has every time extended his investigation over 
a very short period (7 days) which does not seem desirable, if one 
takes into consideration the great variability in the development of 
tadpoles.. Altbough apparently a poisoning in the solutions with a 
greater quantity of uranium-salt does not seem impossible, the fact that 
many tadpoles develop, and that they live quietly on, in much 
stronger uranium solutions, and the failing of uranium in the body, 
might plead against the poisoning of the uranium-salt as a cause 
of the more slow development. 
Microscopic investigation of series sections from some uranium- 
tadpoles, in comparison with normal tadpoles of the same size, has 
not yet shown differences in structure or in degree of development 
of certain organs, which, should be of importance for the growth. 
4) E. Hiascu, Die biologische Wirkung einiger Salze. Zool. Jahrbücher. Band 34. 
