400 - Mulficellular Animals, Especially Man 



exposed scrotal sac, and such a temperature 

 damages the sperm-forming tissues but does 

 not greatly harm interstitial cells. Thus if a 

 testis is transplanted from the scrotum into 

 the body cavity, the interstitial tissues persist 

 but the tubules degenerate. Moreover, the 

 importance of the interstitial tissue is borne 

 out by x-ray experiments. A suitable dosage 

 of x-radiation virtually destroys the tubules 

 of the testis, without much impairment of the 

 interstitial cells. Subsequently the treated in- 

 dividual is sterile but still retains the sec- 

 ondary characteristics of the male. 



Isolation of Testosterone. In 1932, testos- 

 terone, C 19 H 30 O 2 , was isolated from a mas- 

 sive extract of sheep testes; and this steroid 

 compound is generally acknowledged to be 

 the main testicular hormone. This crystal- 

 line compound is so potent that just one 

 hundredth of a milligram (.00001 g) will 

 produce a measurable enlargement of the 

 comb of a capon; in other words, one gram 

 — the quantity extractable from about a ton 

 of fresh testes — will give an appreciable ef- 

 fect in 100.000 birds. 



In addition to testosterone, several other 

 steroid compounds, having a distinctly simi- 

 lar though usually less potent action, have 

 been identified. Some of these androgenic 

 compounds are probably metabolic deriva- 

 tives or precursors of testosterone, since they 

 can be extracted from some tissues and from 

 the urine. Others, however, are purely syn- 

 thetic products that derive their androgenic 

 potency from the similarity of their chemical 

 structure to the natural androgens. 



Puberty. Except for the primary differ- 

 ence in the sex organs proper, there is scarcely 

 any sex differentiation in newborn children; 

 and this lack of differentiation between the 

 sexes persists until the time of puberty. At 

 puberty, however, the interstitial tissues of 

 the testes becomes active, and soon the newly 

 mobilized testosterone begins to take effect. 

 Now the boy becomes broader shouldered 

 and more muscular; the voice deepens and 

 the beard begins to grow; also there is a 

 characteristic growth of hair in the axillary 



and pubic regions; and finally the genitalia 

 proper take on the proportions of an adult. 



THE OVARY AS AN ENDOCRINE ORGAN 



In the female, the counterpart to testos- 

 terone is an ovarian hormone, estrogen, 1 

 which is produced by the Graafian follicles 

 (p. 387). These follicles lie relatively dormant 

 in the ovary until the time of puberty and 

 then they ripen and begin to secrete estrogen. 



The puberty changes of the female body 

 are initiated by the liberation of estrogen 

 from the newly developing follicles. Gradu- 

 ally the body contours lose their angularity, 

 and the pelvis becomes somewhat broader; 

 the breasts begin to develop, and there is an 

 appreciable growth of the uterus and the 

 vagina. But the complete development of the 

 reproductive structures, and the establish- 

 ment of a menstrual cycle, necessitates the 

 intervention of a second ovarian hormone. 

 This is the hormone progesterone, which is 

 produced by the corpus luteum.- 



Isolation of the ovarian hormones entailed 

 the cooperation of biochemists and physiol- 

 ogists in several nations; and all the ovarian 

 hormones (except for relaxin) prove to be 

 steroid componds, somewhat similar in chem- 

 ical structure to testosterone. 



The Ovarian Hormones and the Estrous 

 Cycle. Among most mammals, the female of 

 the species displays a distinct and periodic 

 rhythm of sexual activity; and this so-called 

 estrous cycle is dominated by the ovarian 

 hormones. A dog or cat, for example, comes 

 into "heat," or estrus, only about twice 

 yearly; but such species as rats and mice have 

 a cycle that recurs much more frequentlv. 

 Usually the female is receptive to the male 

 only at the time of estrus; and regularly the 

 Graafian follicles reach maturity and ovula- 

 tion occurs at a definite time in the estrous 

 period. 



1 Estrogen: the one or more hormones produced by 

 the follicles. 



2 Small quantities of progesterone are also secreted 

 by the Graafian follicles; ami some estrogen may be 

 secreted bv the corpus luteum. 



