248 THE CONTINUITY OF THE RACE 



general effects upon sex development. They can induce premature sexual ma- 

 turity in young females and are responsible for the development of secondary 

 sex characters, including the body form, breast development, hair growth, and 

 voice timbre characteristic of women. They play an essential role in the menstrual 

 cycle. As the ripening follicle approaches maturity the estrogens enter the blood 

 in increasing amount and cause the epithelial lining of the uterus to thicken, 

 become more glandular, and more richly supplied with blood. Production of these 

 hormones reaches a peak just prior to ovulation and rises to a second peak about 

 10 days later; but if implantation does not occur, it shortly falls off and only 

 resumes when the next follicle begins rapid growth. The role of the estrogens in 

 the reproductive cycle is to prepare the uterus for reception of the embryo and 

 (in lower animals) to cause the other phenomena of estrus. 



Progesterone 1 is the other ovarian hormone. It is secreted in a temporary endo- 

 crine gland that forms in the ruptured follicle from which the egg was discharged. 

 Some of the follicle cells proliferate to form a mass of yellow pigment-containing 

 cells that fill the cavity of the follicle. This mass of glandular tissue is the corpus 

 luteum (Latin, "yellow body"). If fertilization and implantation occur, the corpus 

 luteum lasts throughout the first 7 months of pregnancy and continues all this 

 time to produce progesterone; otherwise it breaks down and is completely ab- 

 sorbed in about 2 weeks after it appeared. 



Progesterone is the hormone of pregnancy. It has no effect upon the reproduc- 

 tive organs until they have been modified by the estrogens, but then it completes 

 the changes which the estrogens began. The membranous lining of the uterus, 

 already thickened and vascularized, is softened and moistened through stimula- 

 tion of its glands to secrete. A favorable environment for the spermatozoa and 

 the developing embryo is thus created. After implantation progesterone causes 

 physiological adjustments in the body that are related to childbirth and post- 

 natal care. 



The gonadotrophic hormones. The periodic changes in the reproductive 

 cycles are directly produced by the ovarian hormones; these are in turn regulated 

 by gonadotrophic hormones produced by the anterior pituitary. Three of these 

 are known: FSH and ICSH, already discussed in connection with the male, and 

 luteotrophin, which acts only in the female. 



In the female the follicle stimulating hormone (FSH) causes rapid growth of 

 the follicle; the interstitial cell stimulating hormone (ICSH) causes the follicles 

 to produce estrogens. Acting together these two hormones cause the follicle to 

 produce the corpus luteum after ovulation. 2 The estrogen formed after ovulation 

 comes from the corpus luteum under the stimulus of- ICSH. The third gonado- 

 trophic hormone, luteotrophin, acts to sustain the corpus luteum after it is pro- 

 duced, is responsible for the production of progesterone by this gland and may 

 be the same as the lactogenic hormone to be mentioned later. 



Relations of the ovarian and gonadotrophic hormones. The various sex 

 hormones are related to one another in a condition of fluctuating balance; those 

 of the ovary affect production of the pituitary hormones, and the latter influence 



1 Also known as progestin. 



2 ICSH was formerly thought to be solely responsible for corpus luteum formation 

 and was often called the luteinizing hormone. 



