54 WALTER HEAPE. 



For this reason the evidence available is fragmentary, bnt 

 it is remarkably consistent ; and although further researches 

 may, and probably will, show variations in detail, the broad 

 fact of the homology of the internal process of pro-cestrum 

 in all mammals is sufficiently demonstrated. 



This we may summarise as follows : the uterus of all 

 mammals dm*ing the quiescent period is comparatively 

 anaemic, and its mucosa is a thin layer ; it has at that period 

 the appearance of lying fallow. 



During pro-oestrum hypertrophy of the mucosa first takes 

 place, and is followed by congestion, which results usually in 

 the rupture of the superficial vessels and consequent ex- 

 travasation of the blood into the surrounding tissue ; in some 

 cases this extravasated blood finds its way into the cavity of 

 the uterus and thence to the exterior, with either more or 

 less denudation of the superficial mucosa, while in other 

 cases there is no external hsemorrhage, and the extravasated 

 blood is absorbed in situ. While, therefore, neither the 

 discharge of blood nor the extravasation of blood is an 

 essential feature of the pro-oestrum, the hypertrophy and con- 

 gestion of the mucosa is invariably present in all mammals, 

 a condition which we may confidently expect to find also in 

 the lower Vertebrata. 



The Period of CEstrus. 



The period of normal oestrus, as I have stated in the intro- 

 duction to this paper, occurs as a result of pro-oestrum. 



As a rule breeders regard oestrus (the period of desire) as 

 an attendant condition of pro-oestrum rather than as a result 

 thereof ; where there is no discharge evident there is some 

 excuse for this .view, especially as, even when a discharge 

 does occur, oestrus may happen before the discharge com- 

 pletely ceases. CEstrus, however, is possible only after the 

 changes due to pro-oestrum have taken place in the uterus. 

 A wave of disturbance, at first evident in the external 

 generative organs, extends to the uterus, and after the various 

 phases of pro-CBstrum have been gone through in that organ, 



