244 THE CONTINUITY OF THE RACE 



funnel-like, fringed openings that communicate directly with the peri- 

 toneal cavity. The open ends of the oviducts contain smooth muscle, 

 have some freedom of movement, and can be applied to the surfaces of 

 the ovaries. It is probable that shortly before ovulation occurs, they 

 cup themselves over the surface of the ovary in the region of the follicle. 

 Even if the egg is not discharged directly into the oviduct, the currents 

 caused by beating cilia within the funnel tend to carry it in. Rare acci- 

 dents are known, however, in which eggs have escaped into the peritoneal 

 cavity and there been fertilized; or in which the egg from one ovary has 

 traveled to the oviduct of the opposite side, when one ovary and the 

 opposite oviduct had previously been removed by surgery. Once in the 

 oviduct, the egg is carried to the uterus by peristaltic waves of contrac- 

 tion 1 and by the beating of the cilia that line the walls of the tube. 



If copulation has occurred, sperm normally meet and fertilize the egg 

 while it is still in the oviduct. In this event early embryonic develop- 

 ment takes place during the several days required for passage through 

 the oviduct to the uterus. If the egg is not fertilized, it is absorbed in 

 the oviduct or uterus, probably through the agency of white corpuscles 

 that pass through the walls of these organs and ingest foreign particles. 

 In this event, there follow the phenomena of menstruation. 



The ruptured follicle from which the egg has escaped soon fills with a 

 mass of yellowish cells and becomes a gland of internal secretion called 

 the corpus luteum. If fertilization does not occur, the corpus luteum 

 persists for about 10 days and then dwindles away; but if fertilization 

 does occur and the developing ovum becomes implanted in the uterine 

 wall, the corpus luteum continues to grow until it reaches a diameter of 

 about 3 i inch by the middle of pregnancy. The corpus luteum is indis- 

 pensable for successful gestation and plays an important role in the 

 regulation of the female reproductive cycle. 



The uterus is the organ in which embryonic development takes place. 

 It is an unpaired, median structure with very thick muscular walls and 

 a central cavity. The two oviducts open into its inner end, and at the 

 outer end a narrow passage extending through the neck of the uterus 

 communicates with the vagina. The walls of the uterus are lined with 

 a vascular and glandular epithelium to which the embryo becomes at- 

 tached. During pregnancy the uterus becomes enormously enlarged, 

 projecting far up into the abdomen. Birth of the child is accomplished 

 by rhythmic contractions of the smooth muscles of the uterine walls, 

 aided by the voluntary abdominal muscles. The neck of the uterus and the 

 vagina are very distensible, and their openings are eventually stretched 

 sufficiently to permit the passage of the child. The vagina communicates 



1 Peristalsis is the progression of bands of constriction along a tubular organ, pushing 

 the contents in one direction. The intestinal movements furnish a good example. 



