The greater part of the material wherewith the work detailed in this paper was done, was obtained during my stay in Calcutta during the early part of 1891, and I am greatly indebted to the Committee of Management of the Calcutta Zoological Gardens, and especially to Professor D. D. Cunningham, F. R. S., both for assistance in getting the animals required and for facilities placed at my disposal for carrying on my work in that country. Two hundred female Macacus rhesus were gent to me from Muttra and thirty from Benares. A very large proportion of these either had embryos in an advanced stage of development in utero, or had recently borne young; of the remainder many had not yet reached the age of puberty, and when all doubtful cases were excluded, seventeen specimens alone remained which could be used for the purpose of investigating the phenomena of menstruation. Five of these were referable to Stage 1, the resting or intermenstrual stage, and the other twelve to various stages of the menstrual cycle.