246 THE CONTINUITY OF THE RACE 



pituitary, as discussed in Chap. VIII. When the anterior lobe is removed from 

 young male animals, they never become sexually mature. The testes cease to 

 develop, spermatozoa are not formed, and there is no expression of the secondary 

 sex characters. Injections of anterior lobe extract result in normal development of 

 such animals. An excess of the hormones causes precocious sexual maturity; male 

 chicks given repeated injections of anterior lobe extract grow large combs and 

 begin to crow before they are fully feathered. Removal of the anterior pituitary 

 from adults is followed by degenerative changes in the testes; spermatozoa cease 

 to be formed, and the effects of castration appear. 



The gonadotrophic hormones operative in the male are two. The interstitial cell 

 stimulating hormone (ICSH) stimulates growth of the interstitial cells of the tes- 

 tis and thus indirectly causes the production of the androgenic hormones. The 

 follicle stimulating hormone (FSH) stimulates the production of spermatozoa in 

 the male, although it receives its name from its first observed effect, that of 

 stimulating growth of the follicle in the ovary of the female. 



In continuous breeders like man there is a constant output of ICSH and FSH 

 at all times, maintaining a continuous production of androgenic hormones and 

 spermatozoa. In seasonal breeders (the majority of vertebrates) the output of 

 gonadotrophic hormones varies; the testes (and ovaries) are small and inactive 

 except just before and during the breeding season. In many birds temperature 

 and length of day have been found to provide the stimuli for increase in produc- 

 tion of the gonadotrophic hormones and thus govern the time when mating and 

 migration occur. Such birds can be made to breed and to migrate in midwinter 

 by regulation of temperature and the daily ratio of light and darkness to which 

 they are subjected in laboratory cages. 



SEX HORMONES AND REPRODUCTIVE CYCLES IN THE FEMALE 



Sexually mature females of all mammals, including the human species, show 

 rhythms or cycles in the reproductive processes and activities. We have already 

 seen something of these periodicities in the preceding section on the female repro- 

 ductive system, but they require further description. We shall see how they result 

 from alterations in hormone balance. 



The menstrual cycle. In the lower mammals mating occurs only during defi- 

 nite breeding seasons, when the female is said to be in "heat" or estrus (Greek, 

 oistrus, "desire"). Estrus occurs at about the time the ripe ovum is ready to leave 

 its follicle; the external genitalia become swollen and congested, and the female 

 becomes receptive to attempts of the male at copulation. The vaginal membranes 

 thicken and become more glandular, the mammary glands enlarge, and the 

 uterine walls undergo changes preparatory to implantation of the fertilized egg. 

 The uterine lining thickens, becomes more glandular, and its blood supply 

 increases. At the completion of these changes ovulation occurs. If the egg is 

 fertilized and implanted in the uterus, the estrus cycle gives place to the preg- 

 nancy cycle; otherwise the vagina, uterus and mammary glands return to their 

 original state, and the animal becomes sexually quiescent until the start of the 

 next estrus cycle. 



Essentially the same sequence of events occurs in man and other primates, but 

 with certain differences. Here ovulation is not accompanied by estrus, although 



