766 RADIATION BIOLOGY 



be the initial effect less the amount of recovery. This is referred to as the 

 residual effect. The dose of radiation that would be required at any 

 instant to produce an effect quantitatively identical with the residual 

 effect is known as the cumulative dose. Ideally the residual effect should 

 be expressed in terms of a deficiency of an essential substance or in the 

 amount of toxic material that is acting to produce the biological effect, 

 but this is not possible at present because of our lack of information on 

 the radiation-induced chemical and physical changes responsible for the 

 effect observed. 



It is these two factors, latency and recovery, that often make it difficult 

 to determine accurately the immediate effect of a given dose of radiation. 

 The measurement of mitotic delay, for example, depends on the timing of 

 irradiated cells or the making of cell counts at certain intervals of time 

 following treatment. Since recovery may be assumed to be in progress 

 during this period and since the maximum observed biological effect 

 appears some time after the primary changes are induced, the accuracy 

 of our determination of initial and residual effect has definite limitations, 

 and these must not be overlooked in choosing a material and a method 

 and in interpreting results. 



HIGH-ENERGY RADIATIONS 



MITOTIC EFFECTS 



Methods. The type of material selected by the researcher for study 

 determines to a large extent the type of mitotic problem that can profit- 

 ably be studied. The most direct approach to the problem of how radia- 

 tions affect mitosis is offered by observations on individual living cells. 

 Hanging-drop preparations of neuroblasts of the grasshopper, Chorto- 

 phaga viridifasciata, in artificial culture medium are very useful for such 

 studies. These cells are large, the internal structures are clearly visible 

 in the living cell, and all the mitotic stages are readily identifiable. It is 

 possible, therefore, to treat a cell at a known mitotic stage and then record 

 its behavior and time its progress subsequent to irradiation. A present 

 shortcoming of this method is our inability to maintain these cells at a 

 normal rate of division for more than 6-8 hours after the preparation is 

 made up; as a result, experiments designed to test recovery from large 

 doses, which may delay mitosis for several hours, are not feasible. 



The marine invertebrate egg, such as that of Arbacia, also makes 

 possible treatment at a known stage of division and accurate timing of 

 subsequent mitotic progress, but the means of obtaining the data is quite 

 different. All of a lot of eggs from a single female will progress mitotically 

 at about the same rate after fertilization. Since stages in the mitotic 



