452 JOHN W. GOWEN 



interested in the effect of radiant energy on normal cells and cell 

 structures. These studies first investigated the effects of radiant 

 energy on the cell lineage pattern in the unfolding of embryological 

 developments. The studies were confined to particular tissue cells, 

 as for instance the sperm and egg, nucleus, or cytoplasm. The micro- 

 scopic changes observed in these tissues as the exposure progressed 

 were used to measure the radiation effects. Different effects in par- 

 ticular tissues pointed to a selective action of the X rays. The need 

 for understanding radiation effects in the regeneration of particular 

 organs became evident and such investigations soon paralleled those 

 on embryological development. Somewhat later, geneticists became 

 interested in the more intimate phases of X-ray effects within individ- 

 ual cell nuclei. X rays furnished the means for penetrating into the 

 nucleus and exciting particular elements, with the consequence that 

 genes and chromosomes were changed and the course of heredity 

 modified. 



The three types of investigators want very different results from 

 the radiant energy. The first, by using X rays simply as a means of 

 differential diagnosis of abnormal structures, wish X rays to penetrate 

 without producing any obvious effects on the tissues under observa- 

 tion. All that is required is an accurate, contrasting photograph of 

 the normal and abnormal structures. The second group of investi- 

 gators, on the other hand, require the X rays to penetrate to the ab- 

 normal tissues and be absorbed in them to a sufficient extent to cause 

 their death and regression without affecting the normal surrounding 

 tissues. Finally, those who wish to follow the embryological un- 

 folding of organisms through changes produced by radiation, or 

 those in the genetic field who wish to alter genes or chromosomes re- 

 quire the X rays to penetrate to the cells under observation, cause 

 changes in these cells that will lead to the sought-for structural 

 changes in the adult, and yet leave the cells in sufficiently normal 

 condition so that they will go on and reproduce, either to form the 

 complete new body phenotype or to be carried by the germ cells to a 

 following generation, when the altered genes would produce a dis- 

 tinguishable change. 



Besides these main biological interests, X rays have been used 

 for the elucidation of molecular structure. X-ray diffraction photo- 

 graphs of crystals have given information on the arrangement of the 

 atoms within molecules. Similai-ly, in larger molecules it is possible 

 to estimate the geometrical shape and size of molecules from crystal 



