A cytological investigation has been carried out with the ova derived from 66 ovaries which were taken from a series of thirty-three mature unmated mice (Mus musculus, "Swiss albinos") at different phases of the oestrous cycle, with special reference to the maturation and degeneration of ova in mature ovaries.
The atretic follicles of ova which showed advance towards the first maturation division are very abundant in number in the mature ovary, being 85 (35 %) in total. The abnormal ova are mostly at the phase of proestrus in the oestrous cycle. It is most probable that the atretic follicles are in the process of degeneration, and the disintegration takes place either in the egg body or in the granulosa cells.
The polyovular follicles observed in 60 normal ovaries are 80 in total. Out of them 62 follicles (78 %) contain biovular ones. The majority of polyovular follicles are in the process of degeneration, though there are a few examples which are suggestive of the possibility of their maturation.
The atretic follicles are very abundant in three mice which showed an abnormally long dioestrus cycle. From this evidence it is presumable that there is a relationship between the maturation of ova and the effect of the anterior pituitary hormone.