CONNECTION BETWEEN OVULATION AND MENSTRUATION. 463 



two or three days before the commencement of the period, 

 while others maintain that it happens at the middle, or even 

 towards the end, of the period. It is very possible that there is 

 no constancy in this particular respect. 



A still more difficult point remains to be considered. The 

 menstrual decidua is to be viewed as a preparation on the part 

 of the uterus for the reception of an ovum ; but it has still to 

 be determined whether the decidua which is broken up and 

 discharged at a given menstrual period is the one prepared for 

 the ovum which is set free from the ovary at the same period, or 

 for an ovum liberated at some previous or subsequent period. 

 The question is one of great importance, as the means of deter- 

 mining the age of human embryos are very materially affected 

 by the answer given to it. 



The menstrual cycle has been seen to consist essentially in 

 a periodically recurring preparation of the uterus for the recep- 

 tion of an ovum. It is important to determine, if possible, at 

 what particular phase of the cycle, the uterus is in the condition 

 most favourable for the reception of an ovum. Very different 

 views have been expressed on this point, and two of these call 

 for special notice. 



(i) That the end of the constructive period is the natural 

 and most favourable moment for the ovum to enter the uterus. 



(ii) That the period of quiescence is the most favourable 

 time. 



In support of the former view, it is urged that the formation 

 of the decidua is unintelligible except on the supposition that it 

 is a preparation for the reception of the ovum ; and that the 

 analogy of the rabbit's uterus, in which the sequence of changes 

 is strikingly similar, is in favour of the end of the constructive 

 period, or perhaps the commencement of the destructive period, 

 being the one specially concerned with the fixation of the ovum 

 to the wall of the uterus. 



It must be noticed, however, that if the normal time of 

 attachment to the uterus is, in the human ovum, the end of the 

 constructive period, i.e. the commencement of the menstrual 

 period, then it is clear that the ovum which is to be attached 

 cannot be the one discharged from the ovary at the same period. 

 For the discharge of the ovum is practically coincident with the 

 onset of the menstrual period ; and the ovum, after leaving the 



