568 BIOLOGICAL EFFECTS OF RADIATION 



at this stage of development, the eggs were least susceptible to X-rays 

 which, of course, penetrated to all parts of the egg. 



Redfield and Bright (16) have studied a very interesting effect of 

 alpha particles upon the egg of Nereis limbata. Immediately beneath the 

 vitelline membrane of the unfertilized egg is a coarsely alveolar layer 

 about 4/x thick. This layer contains a jelly which, after fertilization, is 

 extruded through the vitelline membrane, forming a thick layer outside 

 the egg. The emptied alveoli become filled with water, forming a thin 

 peri vitelline space between the vitelline membrane and the protoplasm. 

 Irradiation with alpha particles caused a decrease in jelly extrusion, while 

 the volume of the perivitelline space was correspondingly increased. 

 This effect was also produced by beta rays, gamma rays, and ultra-violet 



o 



light of wave-length less than 3000 A and hence seems to be an effect of 

 ionizing radiations in general. 



In a later paper (18) these authors demonstrate a striking relationship 

 between ionizing power of alpha rays and degree of biological effect 

 (thickness of perivitelline space). The source of radiation was an 

 exceptionally thin-walled glass bulb containing radon and, of course, 

 radium A and radium C. The alpha particles from radon, radium A, 

 and radium C have different penetrating abilities. Hence the experi- 

 mental curve describing the variation in ionization per unit path with 

 the distance from such a source is not so simple as that shown in Fig. 1 

 for a polonium source. It has three maxima and two minima. Two 

 of the maxima and one of the minima are quite pronounced. Redfield 

 and his coworker exposed various lots of Nereis eggs to the same dose 

 of alpha particles per egg but with the thin perivitelline spaces at differ- 

 ent distances from the source. When degree of biological effect was plot- 

 ted over distance from the source, the resulting curve had two maxima 

 and one minimum. Moreover, the distances between these three critical 

 points were the same as the corresponding distances on the ionization 

 curve. This result shows very clearly that the biological effectiveness 

 of the alpha particle varies along its path, and that this variation is 

 closely related to the variable ionization per unit path. 



Unfortunately, because of limited data and the heterogeneity of the 

 alpha particles used, it is impracticable to deduce just what mathematical 

 relationship existed between ionization per unit path and the effectiveness 

 in prevention of jelly secretion. If the biological effectiveness could be 

 shown to be directly proportional to the ionization per unit path, then it 

 would be evident that the production of a given degree of effect was due 

 only to the production of a given number of ions. It would be immaterial 

 whether this number of ions be produced by a relatively small number 

 of alpha particles traversing the perivitelline space at that part of the 

 path where the ionization is relatively high, or vice versa. On the other 

 hand, if biological effectiveness could be shown not to be directly propor- 



