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Summary.

The reproductive cycle of the female weasel (Mustela nivalis) has been studied by the examination of the ovaries, uterus, etc., in 114 animals killed in the field. Some additional weasels were kept in captivity for periods varying up to sixteen months. Pregnancies are most common in April or May, but they will occur in any month from March till August. Weasels having their first litter in the spring can breed again in the summer. The majority of weasels do not breed until their second season, but some born in spring become pregnant in late summer. Anœstrus commonly lasts from September till February, but in individual animals the first œstrus may not occur until May or even June. It is estimated that pregnancy and lactation together last about 10 weeks. Histologically, the reproductive organs resemble those of the ferret and stoat. The number of corpora lutea in the two ovaries varies from 4–11, with an average of 7. Six or seven fœtuses were found in the small number of pregnancies in which they could be counted. It seems probable that in the weasel ovulation is dependent upon copulation.

The collection of weasels described in this paper was begun by Dr. A. S. Parkes, F.R.S., in 1930, and later continued by Dr. M. Hill. I am much indebted to Dr. Hill for transferring to me her sectioned material, with extensive records, notes, measurements anti micro-photographs, when circumstances made it impossible for her to complete the work. The micro-photographs, except for two, are the work of Mr. F. J. Pittock.

Rather more than half the female weasels were obtained through the co-operation of Professor F. W. R. Brambell, and his continued help has been greatly appreciated.