Abstract
The effect of light on the time at which sexual maturity is attained and on the sexual cycle of a polyestrous mammal has been studied by Browman 1 and by Hemmingsen and Krarup, 2 but the nature of the change in the secretory activity of the pituitary responsible for these phenomena is as yet incompletely understood. It was with this aspect of the problem in mind that the following work was undertaken.
Immature female rats were kept in darkness, in the ordinary laboratory light, and under continuous lighting from the twenty-first day of life. The rats were examined daily for vaginal introitus. The rats came into sexual activity most quickly when kept in the light, least quickly when kept in the dark (Table I). Fifty rats placed in a dark room and an equal number under light from birth, rather than when twenty-one days old, failed to give results which differed significantly from those just reported.
As soon as the vaginal membrane of the rats kept in darkness or under continuous illumination was found to be ruptured, the rats were examined by means of vaginal smears for changes in the estrous cycle. Rats kept in the light were found to have prolonged periods of estrus and somewhat lengthened periods of diestrus whereas the animals kept in the dark were continually fluctuating between estrus and diestrus and showed longer periods of metestrous smears than the normal rat (Table II).
Pituitaries from females kept in light or in darkness for 100 days were tested for their LH and FSH content by the injection of such material, following crude extraction, into 21day-old male and female rats. It was found that the pituitaries from the females kept in the light were the more potent in producing ovarian development, whereas the pituitaries from females kept in the dark were the more potent in producing seminal vesicle growth.
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