In accordance with what appears to be a general rule among mammals, the ovum of the rat at first grows very rapidly in relation to the follicle of which it is part; it then remains more or less stationary in size while the follicle continues to expand. The slope of the line relating ovular to follicular size in the first phase of growth has been found to differ significantly from rat to rat. The range of size (in stained sections) within which the regression lines for the two phases of follicular growth intersect is 78–153μ. This means that on an average the oocyte reaches its maximum size when it is covered by four layers of granulosa cells, and before an antrum appears.