464 THE HUMAN EMBRYO. 



ovary, has still, in order to reach the uterus, to travel along the 

 entire length of the Fallopian tube, a passage which is known 

 to take three days in the rabbit, and eight to ten days in the 

 dog, and which in all probability takes at least a week in the 

 human species. It follows that the decidua which is discharged 

 at a given menstrual period cannot have been prepared for 

 the ovum discharged at the same period, but must be the pre- 

 paration for the ovum which was discharged at the preceding 

 menstrual period. 



The second view, that the period of quiescence in the 

 menstrual cycle is the most favourable time for the entrance of 

 the ovum into the uterus, leads to the same conclusion, inas- 

 much as the only ovum which could reach the uterus during 

 the quiescent stage is the one discharged at the previous 

 menstrual period. 



In favour of this second view, that the quiescent period in 

 the menstrual cycle is the most favourable time for the ovum to 

 enter the uterus, the following considerations may be urged. 



(a) A much greater range of time is given, within which 

 the uterus is ready for the reception of an ovum. The quiescent 

 period is the longest of the four stages which compose the men- 

 strual cycle, lasting from twelve to fourteen days ; while, on the 

 view that the completion of the constructive process marks the 

 time at which the uterus is best fitted to receive an ovum, the 

 range of time is limited to two or three days at most ; and the 

 longer period is more in accordance with what is known of the 

 range of time within which conception may occur. 



(h) The stages in the formation of the menstrual decidua 

 have been compared, above, with the changes which occur in the 

 uterus of a rabbit, from the fourth to the seventh or eighth day 

 of pregnancy ; and the close similarity between the two cases has 

 been insisted on. It should now be noticed that these changes 

 in the rabbit occur after the entrance of the ovum into the 

 uterus ; i.e. that in the rabbit the ovum enters the uterus while 

 this latter is in the quiescent stage. 



Neither of the above arguments is at all conclusive, and the 

 question is still an open one. It must be repeated, however, 

 that if either of these views is correct, the same conclusion 

 follows with regard to the relation between ovulation and men- 

 struation, viz. that the decidua of a particular menstrual period 



