THE UTERUS DURING MENSTRUATION AND PREGNANCY 231 



there is also an active discharge of secretion from the uterine glands. The 

 surface epithelium and a portion of the underlying tissue may or may 

 not be desquamated. In some cases the surface epithelium and most of 

 the compact layer may be expelled, aided by painful contractions of the 

 uterus. 



In the third stage, the mucosa becomes thin, with straight, narrow 

 glands, between which are fusiform, closely packed stroma cells. Any 

 surface epithelium which has been desquamated is regenerated from 

 the epithelium of the glands, and gradually the mucosa returns to a 

 resting condition, during which, however, there is a slow process of cell 

 proliferation. 



Implantation of the Ovum. The earliest known human ova are al- 

 ready 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, such as 

 the guinea pig, the course of events in man is reasonably 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 ampulla. 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 occupying about eight days, the ovum loses its sur- 

 rounding 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 (von Spee). 



Since ovulation occurs most often in the intermenstruum, Grosser believes that the 

 embryo reaches the uterus during the premenstrual period. The congestion and loosening 

 of the uterine tissue at this time would seemingly favor the implantation of the embryo, 

 and the glandular secretion might afford nutriment for its growth until implantation oc- 

 curs. The first phase of menstruation, according to this view, 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. Menstruation proper would 

 then represent an over-ripe condition of the mucosa and the abortion of an unfertilized 

 ovum. 



If the ovum becomes implanted and develops elsewhere than in the uterus the 

 condition is known as an extraulerine, or ectopic pregnancy. The commonest site is the 

 uterine tube, tubal pregnancy. Attachment to the peritoneum, abdominal pregnancy, and 

 the development of an unexpelled ovum within the ruptured follicle, ovarian pregnancy, are 

 known also. 



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

 trophectoderm supposedly producing an enzyme which digests away the 

 maternal tissues until the embryo is entirely embedded (Fig. 239). Dur- 

 ing implantation, the trophectoderm also absorbs nutriment (chiefly blood) 

 from the uterine mucosa for the use of the embryo. The process of im- 



