L. H. GRAY 



crustacean, viz- the large water flea, Dahpnia magna, to X radiation. When 

 exposed to X radiation at dose rates exceeding 150 r/min, the Daphnids 

 instantly responded by swimming downwards and away from a source of red 

 light, In' means of which they were being examined. On interrupting the 

 irradiation, the Daphnids instantly reversed these two trends in their direc- 

 tion of movement. The change in behaviour pattern is thus initiated by a 

 total dose of only a few rads, provided this is delivered at 150 rad/min. Such 

 behaviour can be produced by high temperature, high pH, or low pressure. 

 The possibility that the effect of irradiation was mediated through one of 

 these agents was excluded, but in the course of the investigation it was found 

 that the swimming pattern varied systematically when dyes of graded Redox 

 potential were added to the water. Compounds with potentials, relative to 

 the hydrogen electrode, of less than +0-045 (Brilliant cresyl blue) simulated 

 irradiation, and six compounds with potentials from +0-062 (Thionine) 

 to +0-275 (Catechol) produced the converse behaviour. The authors 

 suggested that response to irradiation may be due to a radiation-induced 

 reversible reduction of the eye pigment in the nauplius eye, but the alter- 

 native of an induced fluorescence or chemiluminescence in the substances of 

 the eye pigment, or its immediate environment, cannot be said as yet to have 

 been ruled out. 



The optic nerve is stimulated by an appropriate chemical agent in a very 

 small number out of a large array of identical pigment molecules. If the 

 required chemical change is one which can be effected by ionizing radiation 

 — and reversible reduction is a well established radiation-induced trans- 

 formation in methylene blue and many other molecules^"- ^^ — it must be 

 expected that this change will occur in a small proportion of the molecules 

 exposed to a dose of a few rads. 



Other interesting examples of immediate changes in behaviour patterns 

 of animals in response to the onset of irradiation at dose rates of a few 

 hundred r/min have been described by Hug^^'^^. 



In the examples cited above, detection of the primary radiation distur- 

 bance at the molecular level has rested either on the uniqueness of the 

 molecules in question or on the existence within the cefl of a mechanism 

 which responds to a change in a very small proportion of a large aiTay of 

 identical "molecules. The mere loss in effectiveness of a small proportion of 

 identical molecules serving the same function would not be expected to be 

 detectable, and this has often proved to be the case. Some striking examples 

 have been quoted elsewhere^* and include the inability of radiation at dose 

 levels less than about 100,000 r to interfere with the emission of light by 

 luminous bacteria i'' with adaptive enzyme formation i^, or with the ability 

 of bacteria to support virus growth^". The possibility, of course, exists that 

 an altered molecule might disturb biological function not by its loss, but by 

 reason of a positive toxicity. It might act as an anti-metabolite in relation 

 to its own proper function, or it might be endowed with more general toxi- 

 city. In this latter connection the formation of organic peroxides calls for 

 special consideration since peroxides are formed by the irradiation of almost 

 any organic material in the presence of oxygen. The experiments of Dewey 

 and Boag^* indicate that all the oxygen present in a bacillus at the beginning 



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