INTEGRATION OF ACTIVITIES 157 



tion of production and disappearance of certain hormones; to understand 

 these, it is necessary to know the operations of the female reproductive 

 system. The following account is limited to the mammals. 



The female reproductive cells, in different stages, are contained 

 in the ovary. Each cell is surrounded by liquid enclosed in a layer 

 of cells known as the follicle. The cells (one or more at a time) ripen 

 with considerable regularity, every 5 days in the rat, each 28 days in 

 man, tmce a year in the dog. In the maturing of a cell the follicle grows 

 and approaches the surface of the ovary (Fig. 127). The follicle is there 

 ruptured, and the egg escapes into the open end of the oviduct. The cells 

 of the broken follicle become converted into a yellowish mass called the 

 corpus luteum, while the egg moves down the oviduct. If the animal 

 has mated, spermatozoa may have moved through the uterus and into 

 the oviducts, and the egg may be fertilized there. If it is not fertilized, 



Fig. 127. — Human ovary to show follicles and corpora lutea. At left, surface view, 

 with two follicles of different ages protruding. At right, section showing cla, two degener- 

 ating corpora lutea of different ages; civ, fresh corpus luteum;/, follicles; o, ovum. 



the egg disintegrates or passes out to the exterior. If it is fertilized, it 

 sinks later into the wall of the uterus and proceeds to form an embryo. 

 To receive the fertilized egg, the wall of the uterus must become thickened, 

 glandular, and supplied with an extra amount of blood. This prepara- 

 tion is all wasted if the egg is not fertilized, for then the uterine wall 

 recedes to its "resting" condition. The corpus luteum degenerates 

 (in about 2 weeks in man) if the egg is not implanted in the uterus but 

 continues throughout pregnancy if implantation occurs. 



What governs all these events, to ensure that they occur in the proper 

 relation to one another? In general, it is an interplay of hormones from 

 the reproductive organs and the pituitary gland, one gland stimulating 

 the other and then being inhibited when its product increases to a certain 

 concentration. The pituitary, by means of a hormone, stimulates the 

 growth of the egg follicle; the follicle then produces a hormone which 

 induces the thickening of the uterus just described. When the follicle 

 is ruptured, its hormone is no longer produced, but another hormone is 

 produced by its successor, the corpus luteum, which continues the 

 preparation of the uterus. No other follicle is growing in the meantime, 

 for the hormones of the follicle and corpus luteum inhibit the pituitary, so 



