With female ferrets subjected to different degrees of intensity of light irradiation as measured by placing them at different distances from a 1000 W. lamp, the acceleration of the oestrous cycle, generally speaking, was correlated with the degree of intensity. There were, however, individual exceptions. The distances between the ferrets and the lamp varied from 1 to 22 ft.

Female ferrets subjected to ultra-violet irradiation usually remained on heat until much later in the year than those submitted to light irradiation. With male ferrets ultra-violet irradiation caused descent of the testes in December and the testes remained descended until the autumn.

Irradiated female ferrets (both with light and ultra-violet rays) went off heat shortly after being put with males and probably as a result of copulation (which, however, was not always observed), but pregnancy did not supervene. This may have been due to the animals not being in a state for ovulation.

Feeding vitamin D to anoestrous ferrets did not result in accelerating the cycle, the ferrets not coming on heat until the normal time. The treatment, however, resulted in their becoming abnormally fat. Vitamin D was given in a commercial preparation which also contained vitamin A.

Sections through the ovaries of irradiated oestrous ferrets showed large ripe follicles and other follicles in varying degrees of development. Some apparently unruptured degenerate follicles were also seen. There were no cystic follicles in any of the ferrets' ovaries observed. Typical interstitial cells were present usually in great abundance.

In a ferret that had been submitted to ultra-violet irradiation the ovaries contained a quantity of old luteal cells, the presence of which suggested that some of the follicles had become lutealized without rupturing.

The uterus in three of the irradiated ferrets was somewhat congested and showed signs of glandular activity, being, generally speaking, similar to that of normal ferrets which have been on heat a long time. The uterus of a female ferret, in which the ovaries contained old luteal cells, presented the appearance of one belonging to an animal that was coming on heat again after pregnancy or pseudo-pregnancy.

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