Chap. 18 REPRODUCTION 351 



Two features of the typical reproductive cycle of the lower mammals are 

 especially significant. ( 1 ) Ovulation occurs at a time of sexual excitement, and 

 mating will take place only during that period. (2) The degeneration of the 

 corpora lutea and the withdrawal of preparations for an embryo in the uterus 

 cause very little physiological stir. 



Reproductive Cycle. As already noted, the reproductive cycle of men- 

 struating mammals (the human species and the closely related apes and higher 

 monkeys) is similar to that of other mammals except for activities associated 

 with ovulation and the breakdown of the uterine lining. 



The changes in the ovary including ovulation proceed as in other mammals. 

 Usually, only one egg follicle enlarges and finally breaks, releasing its egg and 

 the estrogen it contains. Ordinarily, the ovary is already clasped by the ciliated 

 funnel of the oviduct and the egg is at once drawn into it (Figs. 18. 11, 18.14). 

 As already stated, even before it leaves the follicle, the number of its chro- 

 mosomes has been reduced from the 48 of the general human body cells to 

 the 24 of the human sex cells. The egg is carried slowly along the oviduct by 

 the currents created by cilia and by the contraction of its muscle. In other 

 primates, fertilization occurs in the oviducts, and this is known to be true 

 of the human egg. Stimulated by the pituitary (Fig. 18.13), the enlarging 

 follicle steadily secretes estrogen into the blood up to the time when the follicle 

 releases the egg. Stimulated by this estrogen, the lining of the uterus becomes 

 more glandular. After ovulation, the corpus luteum provides the progesterone 

 which further induces the enlargement of the uterine glands, their secretory 

 activity, and the increased blood supply (Figs. 18.11, 18.12, 18.14). All of 

 these changes reach their height in the second week after ovulation. If an 

 embryo arrives in the uterus at this time, it is surrounded by ideal conditions 

 for its reception and nourishment. The embryo is extremely minute. As in 

 the pig it produces important reactions in the uterine wall, and substances 

 are absorbed into the blood that prolong the existence of the corpus luteum. 

 As in the pig, too, the placenta provides hormones which help to stimulate the 

 uterus to hold the embryo. 



A very different program follows if no embryo enters the uterus although 

 the latter is highly prepared for one. The corpus luteum degenerates for want 

 of stimulation via the blood from the uterus. Cut off from progesterone, the 

 uterus goes through the violent reactions of menstruation. Its swollen blood 

 vessels are disturbed and ruptured; its lining cells, glands, and inner connective 

 tissue break down. Blood from the broken vessels is mixed with the sloughed 

 off tissues, and the whole cast off debris is gradually drained away through 

 the vagina, a process lasting from one to seven days, but most often for five. 

 Even in the latter part of the period another ovarian follicle is already forming 

 and under the influence of its secreted estrogen the lining of the uterus and its 



