ACTION OF VISIBLE RAYS OX ARBACIA EGGS. 9 



have shown themselves to be more toxic in comparable con- 

 centrations than eosin and benzoflavine even in the dark. (See 

 also MacArthur, '24.) That they also possess photodynamic 

 properties is evident from the fact that their toxicity, in weak 

 concentrations, is augmented in the presence of light. (See 

 Cooke and Loeb, '08, Bohn and Drzewina, '23, O. Hertwig, '12.) 

 The present experiments have shown that methylene blue in 

 comparable concentrations is about thirty times as toxic as eosin. 

 An exposure of approximately fifteen times the duration is 

 required to produce the same degree of inhibition with eosin as 

 with one half the concentration of methylene blue. This is to be 

 related to the position of the absorption band of methylene blue, 

 which is near the region of maximum emission in the energy 

 spectrum. 



Eosin and benzoflavine have long been known as photodynamic 

 sensitizers. Benzoflavine was found by Viale, '19, to be only 

 slightly effective. On the other hand, eosin has been most 

 commonly used as a sensitizer. 1 Eosin absorbs in the blue-green 

 region, neutral red in the blue and blue-green, methylene blue in 

 the orange and red, and benzoflavine in the azure-violet of the 

 visible spectrum. Sensitization with these dyes makes tissues 

 sensitive to the action of rays of a wave-length corresponding to 

 the absorption band of the particular sensitizer used. 



In comparing the relative effectiveness of the various light 

 sources, it may be noted that with 1/10,000 eosin (30 minutes' 

 staining), exposure to radiation from the photographic bulb was 

 90/7 times as effective in producing 100 per cent, inhibition as 

 exposure to sunlight. This estimate is based on a ratio of the 

 periods required in each case to produce inhibition in all 'eggs. 

 Temperature was controlled. 



Conclusions. These experiments, in which the embryonic de- 

 velopment of Arbacia was modified by means of visible radiation 

 following sensitization, offer further evidence for the following: 



i. The effectiveness of photodynamic sensitization depends 

 upon (A} exposure of the sensitized system (in this case, fertilized 

 eggs) to radiation of sufficient intensity and duration (dosage), 



1 See Ledoux-Lebard, '02, Hertel, '05, von Tappeiner, '07, Loeb, '070, b, Cooke 

 and Loeb, '08, '09, O. Hertwig, '12, Schanz, '18, Clark, '22, Metzner, '21, '23, 

 Lillie and Hinrichs, '23, Viale, '21, '23, '24, Hinrichs, '24, and Pereira, '25. 



