THE UTERUS DURING MENSTRUATION AND PREGNANCY 23 1 



The premenstrual changes of the first phase are regarded as the most im- 

 portant part of the cycle, the uterine mucosa thereby being prepared for the 

 reception of a fertilized ovum and for the development of the decidual mem- 

 branes. Menstruation proper, as seen in the second phase, is the result of an 

 over-ripe condition of the mucosa and has been regarded as the abortion of an 

 unfertiKzed ovum. 



The Implantation of the Ovum. — The earliest known human ova are already 

 completely embedded in the uterine mucosa. From the careful study of early 

 human embryos by Bryce and Teacher, Peters, Herzog, and others, and from 

 more complete observations on other mammals (e. g., guinea-pig), the course of 

 events in man is tolerably certain. 



Ovulation sets the ripe ovum free within the abdominal cavity, from whence 

 the beating cilia on the fimbriae of the uterine tube sweep it into the tubal am- 

 pulla. There it may be fertilized and carried to the uterus by the cilia of the 

 tubal epithelium. During this period of migration, which is estimated as occu- 

 pying about eight days, the ovum loses its surrounding follicle cells and pellucid 

 membrane and begins its development. Thus when it reaches the uterus, and is 

 ready for implantation, it is an embryo with trophectoderm developed although 

 the blastodermic vesicle is not more than 0.2 mm. in diameter (Graf Spec). 



If ovulation precedes menstruation proper by ten or twelve days, as Ancel and Villemin 

 maintain, then the embryo would reach the uterus during the premenstrual period. The con- 

 gestion and loosening of the uterine tissue at this time would favor the implantation of the 

 embryo and the glandular secretion would afford nutriment for its growth until implantation 

 occurs. The first phase of menstruation according to this view, that of Grosser, prepares the 

 uterine mucosa for the reception of the embryo. If pregnancy supervenes, it soon inhibits 

 any further premenstrual changes so that menstruation does not occur. 



The embryo penetrates the uterine mucosa as would a parasite, the trophec- 

 toderm supposedly producing an enzyme which digests away the maternal tissues 

 until the embryo is entirely embedded (Fig. 239). During implantation, the 

 trophectoderm also probably absorbs nutriment from the uterine mucosa for the 

 use of the embryo. The process of implantation is supposed to occupy one day. 

 At the point where the embryo enters the mucosa a fibrin clot soon appears and 

 eventually the opening is completely closed (Fig. 239). 



The Decidual Membranes (Figs. 240 and 241). — ^With the increase in size 

 of the embryo and chorionic vesicle, the superficial layers of the maternal mucosa 

 bulge into the cavity of the uterus and form the decidua capsularis (old term, de- 

 cidua refiexa) . The deep layer of the mucosa on the side of the embryo away from 

 the uterine cavity forms the anlage of the future maternal placenta and is the 



