The ovum of placental mammals completes its growth long before the follicle, in which an antrum develops only after the ovum has reached full size. In the first quantitative demonstration of this fact, Brambell [1928] showed that the development of the follicle relative to that of the oocyte may be divided into two phases. In the first the regression line relating ovular to follicular size is very steep, and in the second it is practically horizontal. The point at which the two lines intersect gives the approximate time, in relation to follicular size, at which the ovum completes its growth. Measured in these terms, the time is much the same in different species, a fact which is related to the comparatively small differences in the size of the mature ovarian oocyte in all placental mammals. Thus, the diameter of the follicle at the end of its first phase of growth