RADIATION AND ARBACIA GERM CELLS. 459 



distance of 25-50 cm. above the experimental material. In 

 some experiments, a second lamp burning tip-up was simul- 

 taneously used at the same distance below the experimental 

 material. Temperature was controlled by the addition of ice to 

 the water-bath surrounding the dishes. A glass dish containing 

 a one-inch layer of water filtered out the injurious ultraviolet 

 and heat rays. 



2. A loo-watt condensed-filament lamp (of the type used with 

 the Bausch and Lomb euscope) running at 6 volts, was placed 

 at a distance of 25 cm. from the substage mirror of a microscope, 

 with a water filter interposed between the lamp and the mirror. 

 The dish containing the experimental material was placed on 

 the microscope stage and samples of eggs or sperm were with- 

 drawn at intervals from the center of the region reached by the 

 light reflected up through the condenser and dish. 



3. In some experiments a carbon arc, water filter, and glass 

 lens were used in place of the above arrangement. Samples were 

 withdrawn from the lighted portion at the focal point of the lens. 



4. Direct sunlight (on mid-August afternoons) filtered through 

 a one-inch layer of water in a glass dish was used in another set 

 of experiments. A water bath surrounding the experimental 

 dishes further controlled the temperature. 



5. Diffuse daylight near a north window was used in another 

 series. The dishes were covered with -glass plates. 



In all experiments, eggs or sperm were exposed in a thin layer 

 just covering the bottom of the dish (about 2 mm. deep). The 

 maximum effective periods of exposure and the intervals between 

 the removal of successive samples were necessarily shorter with 

 the intense radiation of the artificial light sources, than with 

 direct sunlight or diffuse daylight. Typically, eggs fertilized by 

 sperm which had been exposed for 45 minutes to direct sunlight 

 showed merely a differential acclimation following a slight inhi- 

 bition ; while when sperm was used which had been exposed for 

 only 5 minutes (under the above conditions) to radiation from 

 Tungsten bulbs or the carbon arc, the result was differential 

 inhibition with no recovery in a large proportion of the embryos. 



In the experiments with visible radiation, stained and un- 

 stained sperm (or eggs) were radiated, and a parallel unradiated 



