1. In the presence of autogenous red blood cells, fibroblasts, grown in vitro from the heart and intestine of embryo chicks of from 6–12 days incubation, underwent a rapid degeneration when exposed to light of the visual spectrum.

2. In this degeneration, the cells showed a marked increase in refractive index, and a massive formation of colorless vacuoles of low refractive index. These vacuoles gave no reaction with osmic acid or Sudan III, but took up neutral red. Later in the degeneration the cells showed marked signs of rounding up and of coagulation.

In some cases this degeneration process was modified in such a manner that, instead of the formation of vacuoles, the whole cytoplasm became much less viscid and the cells swelled greatly and became spherical.

3. The similarity of this degeneration to that shown by the erythrocytes, and more particularly, to that shown by the polymorphonuclear neutrophils, under the action of light, is pointed out.

4. Degeneration of the fibroblasts occurred when the cells were irradiated by light of any one of the following wave-length zones: (a) 430µµ–550µµ; infra-red; (b) 475µµ–630µµ; 690µµ-infra-red; (c) 60µµ-infra-red.

5. This degeneration was much slower for the cells irradiated through the blue or red filters of less than 5 per cent, and less than 6.6 per cent, respective total transmissions. When irradiated through the green filter of 23 per cent total transmission it was much more rapid, and even more rapid than for cultures irradiated with white light of 100 per cent total transmission. These data were obtained on a short series of cultures and must be considered only as suggestive.

6. In the apparent absence of red cells, the fibroblasts underwent only a slight degree of degeneration after irradiation lasting from 13–24½ hours. It is possible that even this minor degeneration may depend upon the presence of traces of disintegration products of red cells in the culture. Much more advanced degenerative processes were obtained, when many red cells were present, after only 3 hours irradiation.

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