IMPREGNATION. 



845 



the development of the allantois it presents three distinct anatomical por- 

 tions : a portion which becomes constricted off, as it were, from the rest and 

 forms the urinary bladder ; the outer portion forms the chorion, the inter- 

 mediate portion forming the umbilical cord. 



FIG. 225. 



FIG. 225. Entire Human Ovum of Eighth Week ; sixteen lines in length (not reckoning the 

 tufts) ; the surface of the chorion partly smooth and partly rendered shaggy by the growth of tufts. 



FIG. 226 Portion of One of the Foetal Villi, about to form part of the Placenta; highly magni- 

 fied, a, a, its cellular covering ; b, b, b, its looped vessels ; c, c, its basis of connective tissue. 



During the development of the embryo up to this time, the first chorion 

 was formed by villosities formed on the vitelline membrane ; and following 

 that by villosities developed upon the false amnion. The allantois then 

 becoming developed, completely covers the internal surface of the false 

 amnion, which then gradually disappears as a distinct structure. The true 

 chorion is then formed by the allantois, which becomes covered by a growth 

 of a multitude of vascular shaggy tufts or villi (Fig. 225). These villi, at 

 first, are distributed over the entire surface of the organ, but they soon com- 

 mence disappearing, except at a small area corresponding to the attachment 

 of the pedicle which connects the allantois with the embryo. At this point 

 they become greatly increased in number, and also in size and vascularity. 

 These villi are composed of a fibro-granular matrix, in which are numerous 

 capillary loops, and are covered with a layer of epithelial cells (Fig. 226). 

 This portion of the chorion forms the foetal portion of the placenta. 



No sooner, however, have these changes begun in the ovum than correla- 

 tive changes, brought about probably by reflex action, but at present most 

 obscure in their causation, take place in the uterus. The mucous membrane 

 of this organ, whether the coitus resulting in impregnation be coincident 

 with a menstrual period or not, becomes congested, and a rapid growth takes 

 place, characterized by a rapid proliferation of the epithelial and subepithe- 

 lial tissues. Unlike the case of menstruation, however, this new growth does 

 not give way to immediate decay and hemorrhage, but remains, and may be 



