Although the mouse, Mus musculus, has been used for embryological purposes for nearly a century, no exact knowledge of its oestrous cycle is available. One reason for this is that the mouse, like other rodents, comes into'heat'and receives the male within from six to twenty-four hours after parturition, and most investigators have timed their collection of embryological material from this' heat'moment. Another reason for our lack of knowledge of the oestrous cycle in this form is the fact that external signs of'heat'have been relied upon for diagnosis. Concerning this, Heape states:" It is difficult to determine the length of the prooestrum and oestrus in rodents, since the external signs which characterize these conditions are comparatively