482 J O H N W. G O W E N 



if rare mutations are occurring in a random manner, the difference 

 between the different strains should be highly significant. This 

 analysis will be useful in determining the prior condition of the culture 

 under irradiation. Bacteria from the different strains may now be 

 irradiated with a selected dose. If a given dose of X rays, e.g., 5000 

 r., is producing the mutants, the bacterial strains should be similar 

 in the number of mutants observed. Furthermore, if irradiation is 

 given in increasing doses the increase in number of mutants should be 

 similar for the different strains for each successive like increase in X- 

 ray dosage. Experimental methods of this general design allow the 

 investigator to contrast the hypothesis of so-called adaptive response 

 of bacteria to the environment with that of randomly occurring muta- 

 tions. 



Radiation experiments may have as their objective the production 

 of mutated strains that will be more efficient in the formation of end 

 products. Such experiments were illustrated in the necessity for 

 better penicillin. Starting from the typical culture of the fungus 

 producing but few units per milliliter of penicillin under standard 

 conditions, successive radiations and collection of mutant types vary- 

 ing in the right direction raised the imit production of penicillin to 

 761 units per milliliter. 



5. Embryological Studies 



X-ray techniques have proved useful in many embryological 

 investigations. Two with somewhat different purposes will be men- 

 tioned: investigation of the nature of the alterations produced by 

 radiation within the cell, and plotting the course of embryological 

 development by means of changes induced by radiation in the develop- 

 ing organism. Observations on the effects of X rays largely depend 

 on microscopic stucty of living cells as radiation progresses. A tiny 

 beam of radiant energy is directed on the cell. Progressive changes 

 in the cell occur as irradiation is increased. The first noticeable ef- 

 fect is an increase in visible fat droplets. This change is not fatal if 

 radiation is stopped at this stage. The protoplasm becomes turbid 

 and granular, evidence for denaturation. This stage is, also, not 

 necessarily fatal if radiation is stopped. Further radiation increases 

 cell permeability to neutral dyes, e.g., neutral red. Then coagulation 

 of the proteins actually begins. Protoplasmic streaming is increased, 

 and then decreased. Viscosity of the cell decreases, and then in- 



