ZOOLOGY AND HUMAN WELFARE 



as the technique develops through experimentation on 

 the higher mammals. 



A most remarkable series of discoveries has resulted 

 from recent investigation of the gonads — the testes in the 

 male and the ovaries in the female; and here we have most 

 decidedly an advancing frontier of zoology, for the facts 

 already found point tov^ard hitherto unsuspected com- 

 plexities and bear directly upon functions of the greatest 

 practical interest. The sex hormones certainly control the 

 development of the secondary sexual characteristics, such 

 as bodily size, strength, and shape; the distribution and 

 length of hair; the quality of the voice; and probably 

 certain psychological traits. The differences which natu- 

 rally distinguish the sexes and play such an important 

 part in sex attraction, mating, and reproduction are absent 

 or imperfectly developed if the sex hormone is not produced 

 at the normal time. There is little doubt that some cases, 

 at least, of perverted and otherwise abnormal sex behavior 

 depend upon endocrine disturbances, and it is equally likely 

 that further knowledge of these conditions will lead to 

 practical methods of cure. But there is much more than 

 this. 



In the female the ovary produces several hormones. 

 One, as we have seen, governs sexual development; another, 

 produced by the corpus luteum (a group of cells formed 

 on the surface of the ovary as each egg is extruded), 

 controls menstruation and affects the condition of the 

 breasts most strikingly when pregnancy occurs. If the egg 

 fails to develop in the uterus, the corpus luteum soon 

 degenerates, menstrual bleeding follows, and the mammary 

 glands are little affected. But when the fertilized egg is 

 implanted and forms an embryo, the corpus luteum persists 

 and grows larger, probably preventing menstruation and 

 giving out the hormone that stimulates growth and 

 activity in the breasts. There are obscure relations here 



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