THE UTERUS DURING MENSTRUATION AND PREGNANCY 239 



struation proper, as seen in the second phase, is the result of an over-ripe condi- 

 tion of the mucosa and has been regarded as the abortion of an unfertilized ovum. 



THE DECIDUAL MEMBRANES: PLACENTA 



The Implantation of the Ovum. Our knowledge concerning the implantation of the 

 ovum is fragmentary, but certain facts have been deduced from observations on mammals 

 (hedge-hog and guinea-pig) , and from the careful study of early human embryos by Teacher, 

 Bryce, Herzog and Peters. The embryo described by Teacher and Bryce, while it is the 

 youngest yet observed, is perhaps not normal. 



After ovulation, the ripe ovum is set free within the abdominal cavity, from 

 whence by the beating cilia on the fimbriae of the uterine tube it is carried into the 

 ampulla of the latter. There it may be fertilized and is swept into the uterus by 

 the cilia of the tubar epithelium. During this period of migration, which is esti- 

 mated as occupying from five to eight days, the ovum loses its surrounding fol- 

 licle cells and its membrane and begins its development. Thus when it reaches 

 the uterus, and is ready for implantation, it is an embryo with trophectoderm 

 developed but still 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 a ferment which digests away the maternal tissues 

 until the embryo is entirely embedded (Fig. 232). 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. 



The Decidual Membranes (Figs. 233 and 234). 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 reflexa) . 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 

 decidua basalis (d. serotina). The mucosa lining the rest of the uterus is differ- 

 entiated into the decidua vera (parietalis of Bonnet). 



Differentiation of the Trophectoderm. The chorion is at first composed of 



