460 THE HUMAN EMBRYO. 



epithelium is restored, by growth from the lips of the deeper 

 parts of the uterine glands ; and the swelling of the mucous 

 membrane subsides. 



(iv) The fourth stage is the period of quiescence, during which 

 the uterus, having regained its normal structure, remains with- 

 out further change until the commencement of the next suc- 

 ceeding constructive stage. 



The actual and relative durations of the several stages 

 enumerated above are not determined with certainty, and are 

 subject to individual variations. It will, perhaps, be right to 

 assign about a week to the constructive stage ; rather less than 

 a week (five days on an average) to the destructive stage ; three 

 or four days to the stage of repair ; and twelve or fourteen days to 

 the quiescent period ; the four stages together occupying the 

 twenty-eight days which make up the normal menstrual cycle. 



Of the above four stages, the first and second require further 

 attention ; the fourth stage is the normal condition ; and the 

 third stage is merely the return of the uterus to the normal con- 

 dition after a period of disturbance. 



Concerning the first or constructive period, there is hardly 

 any room for doubt that it is to be regarded as a preparation on 

 the part of the uterus for the reception of an ovum. 



The several stages of the process correspond closely, in 

 essential respects, with those that occur in the placental lobes ot 

 the rabbit's uterus from about the fourth to the eighth day. In 

 the rabbit, as in the human uterus, there occur swelling of the 

 mucous and submucous tissues, increased vascularity, a large 

 increase in the number of the connective-tissue cells, and a great 

 enlargement of the uterine glands, which become larger, wider, 

 and more freely branched. These changes, in the rabbit's uterus, 

 are clearly related to the nutrition of the embryo, for it is to this 

 hypertrophied and modified area of the uterine mucous membrane 

 that the embryo becomes attached on the eighth day ; and it is 

 from this area that the maternal part of the placenta is formed. 



The most important difference between the rabbit's and the 

 human uterus, as regards these stages, is that in the rabbit the 

 ovum, or rather the blastoderrnic vesicle, is present within the 

 uterus during the whole of the series of changes, although it lies 

 quite freely and does not acquire attachment until the eighth 

 day ; while in the human uterus, on the other hand, the men- 



