In our previous paper (1934) it was shown that the recurrence of oestrus in the female ferret is greatly accelerated by irradiation with light, and is retarded by darkness, thus confirming and extending the observations of Bissonette (1932). It was also shown that heat rays and the near infra-red (λ7500Å.) were inactive. The active radiation extended throughout the spectrum from the red (λ 6500 Å.) to the ultra-violet (λ3650Å.). Further, the animals irradiated with ultra-violet light remained on heat far longer than any of the others, although the irradiation had been for a long time discontinued. Of the two ferrets which are recorded as having been still on heat on August 1, 1933, one (No. 2) remained in a state of full oestrus until August 28, but the vulval swelling did not finally disappear until October 2, while the second ferret (No. 14) began to go off heat on September 11, but was not completely anoestrus until November 14.

In the experiments commenced in the autumn of 1934 and continued during 1935, attempts were made (1) to repeat and confirm the results previously obtained with ultra-violet light, using the same ferrets as well as two additional ones, (2) to test the effects of heat rays of long wave-length, (3) to compare the effect of daily irradiation with light (a) of high intensity for a short period and (b) of low intensity for a long period. The period and intensity were so arranged that the total quantity of light received daily was approximately the same in each case.

With all the ferrets irradiation was begun on December 1, 1934, and ended on May 14, 1935. The experimental arrangement was similar to that described in the earlier paper.

Filtered ultra-violet light (λ 3650Å.)—intensity at centre of cage ca. 100 ergs cm.2—was switched on each afternoon at dusk and continued for 8 hours. Four ferrets were treated, Nos. 2 and 14 of the original ultra-violet series, No. 9 treated in 1933 and 1934 with red rays, and a new ferret not previously treated (No. 15). No. 2 began to come on heat on February 27, 1935 (an acceleration of 57 days), was fully on heat on March 20, and remained on until August 28, but was not completely anoestrus, as shown by the vulva, until October 2. No. 14 came on heat on February 13 (an acceleration of 71 days) and remained on until it died on April 17.

No. 9 came on heat on February 27 (an acceleration of 57 days) and remained on until it died on June 12. No. 15, the new ferret, came on heat on February 27 (an acceleration of 57 days) and remained in an oestrous condition until it was killed on May 29. The cause of death was in each case uncertain, but all the ferrets were in apparently good health until a few days before they died, except one that suffered from footrot. It will be seen that the experiments confirmed the observation of the previous year that ferrets irradiated with ultra-violet rays came on heat early and remained on heat for a very long period after the stimulation had stopped. The oestrous period was therefore considerably longer than that shown by animals stimulated by luminous rays, or that of unirradiated ferrets.

Coils of nichrome wire heated to about 400 ° C. (just below red heat), were mounted above the ferrets at a distance of 25 cm. The intensity of the heat radiation1 at the floor of the cage was over 100 ergs cm.2 The ferrets did not appear to experience any discomfort. Two ferrets were treated and they both began to come on heat on March 27, which is about the normal time for the commencement of oestrus. One ferret (No. 11) was fully on heat, as shown by vulval swelling, by April 10, and the other (No. 21) by April 17. Both ferrets remained on heat till August 14, when the vulva subsided in the normal way. Apparently heat and infrared radiation have no stimulating effect.

This experiment was made to investigate whether an irradiation with light of low intensity over a long period would have the same effect as light of high intensity for a short period.

(a) Two ferrets (Nos. 3 and 23) were subjected for 2 hours every evening to the full radiation from eight lamp bulbs (each 60 watt). The lamps were 36 cm. away and the intensity of radiation at the floor of the cage was ca. 116 ergs cm.2

(b) Two more ferrets (Nos. 1 and 22) were irradiated for 16 hours every night with a single lamp (60 watt). The intensity at the floor of the cage was 14 ergs cm.2, which is approximately one-eighth of that used in (a).

Both pairs of ferrets thus received the same total quantity of radiation per day, but in one case it was concentrated and in the other spread over several hours.

Of the two ferrets under (a) one (No. 3) began to come on heat on January 23 (an acceleration of 92 days) and was fully oestrous by February 27; the other (No. 23) began on March 6 (an acceleration of 50 days) and was fully oestrous by March 20. Of the two animals under (b) one (No. 22) began to come on heat on January 23 (an acceleration of 92 days) and came on rather quickly, being fully oestrous by February 6; while the other ferret (No. 1) began to come on heat on February 27 (an acceleration of 57 days) and was fully on by March 13. It will be seen that there was very little difference between the times for coming on heat in the two groups. Once oestrus had begun, the ferrets with the extended duration of light developed rather more rapidly than the others.1

It remains to be added that the blind ferret referred to in our previous paper again failed to come on heat and was in a continuous state of anoestrus until July 10, 1935, when it was killed. The ovaries were reduced in size and contained small degenerate follicles. The uterus also was slightly atrophic.

Since our former paper appeared (1934) Bissonette (1934) has published an account of his later experiments which show that ferrets coming into oestrus might undergo regression or retardation as a result of reduction in the intensity or extent of the daily illumination, while in some cases oestrous activation was resumed when the amount of light was again increased. The more recent literature of this subject is referred to and discussed in this paper. That the oestrous cycle in the ferret is considerably influenced by the quantity of light to which the animals are exposed is now definitely established, and it can hardly be doubted that the same principle is applicable to many other species.

It remains to refer to the papers bearing on this subject which have been published since Bissonette’s latest contribution. Benoit (1935) has shown that light has a pronounced effect in stimulating the testes of the drake, and that both immature and mature birds are affected. When a hood was used to cover the eyes the effects did not supervene. Collin (1935) has described growths of nerve fibres going to the pituitary body from the supra-optic nucleus, central grey substance and tuber. New retino-pituitary connections (photo-pituitary fibres subserving pigmentary functions and related anatomically to the thalamus) are regarded as affording an explanation of the mechanism whereby light can influence the sexual processes. Kylin (1935) also has described a path from the third ventricle through the hypophyseal peduncle to the anterior pituitary, the duct being lined with ependyma. That the anterior pituitary is the chief regulating organ of the sexual processes as well as of other metabolic processes is now generally admitted, but Loewi (1935), in his recent Ferrier lecture, speaking in the absence of other definite knowledge, expresses the view that the gland receives its information, not by nerves, but almost exclusively by humoral means.

The investigation of the effect of radiation on the oestrous cycle of the female ferret has been continued.

  1. The earlier experiments with the ultra-violet light have been repeated and confirmed. Irradiation with light of λ 3650Å. caused the ferrets to come on heat early and to remain on heat for an abnormally long time after irradiation had ceased.

  2. Heat rays of long wave-lengths caused no acceleration. This is in accordance with earlier experiments which showed that heat rays and infra-red (λ 7500 Å.) had little effect.

  3. Two pairs of ferrets were subjected to the same total quantity of radiation from incandescent lamps, but in one case it was concentrated into 2 hours, and in the other spread over 16 hours. There was little difference in the behaviour of the two pairs and both showed an acceleration of oestrus.

We would again express our indebtedness to Mr J. Pike.

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1

The intensities were measured with a quartz screen in front of the thermopile so that the very long heat radiation was not included.

1

The “normal” time for coming on heat is taken as being the same as that recorded in our previous paper, viz. 146 days from December 1. The results for the normal control ferrets during the present year fully confirmed the previous observations, the animals coming on heat at the end of March or beginning of April.