462 THE HUMAN EMBRYo. 



preparation of the uterus for the formation of a placenta ; stages 

 which can be carried up to a certain point without needing the 

 stimulus of the presence of an ovum or embryo, but which, 

 ] laving reached a point at which further development is impos- 

 sible without an embryo, stop abruptly. The constructive stage 

 has been shown to be an active preparation of the uterus for the 

 reception of a fertilised ovum; the succeeding or destructive 

 stage is not to be regarded as a simple undoing of this prepa- 

 ration, but as a further continuance, in a modified form, of the 

 act of preparation, which leaves the uterus in a condition in 

 which, for further elaboration to occur, the presence of an embryo 

 is indispensable. 



6. The Connection between Ovulation and Menstruation. 



Ovulation and menstruation, or the discharge of ova from the 

 ovary, and of the disintegrated decidua from the uterus, are 

 processes which occur periodically, and as a rule simultaneously ; 

 and it becomes a matter of interest to inquire into the nature of 

 the connection between them. 



The ovaries swell up, and become tender, at monthly intervals. 

 The enlargement commences, as a rule, a few days before the 

 menstrual period, attains its maximum about the time of the 

 period, and gradually subsides after the period is over. 



As the ovary is known to become congested just before the 

 rupture of a Graafian follicle and the discharge of an ovum, it 

 appears a fair inference that this discharge occurs about the 

 same time as the menstrual flow, i.e. that ovulation and men- 

 struation are practically simultaneous. However, although 

 this may be, and probably is, the rule, yet it is far from being 

 an invariable one. Thus Kolliker, on examining the ovaries of 

 seven women who had died directly after menstruation, found 

 that in two of the cases there was no fresh corpus luteum 

 in either ovary ; that is, that no ovum had been discharged 

 at the time of menstruation ; and Coste has cited similar 

 instances. 



Ovulation and menstruation may be assumed to occur as a 

 rule about the same time, but it is by no means clear what is 

 the precise nature of the connection between the two processes. 

 Authorities differ as to the stage in the menstrual period at 

 which ovulation occurs, the majority holding that it takes place 



