150 READINGS IN BIOLOGICAL SCIENCE 



the pubis symphysis before parturition and thus facilitates bearing the 

 young. The same or similar relaxative hormone has also been recovered 

 from the placenta, or nutritive disk by which the fetus is attached, and 

 from the blood and urine of pregnant animals. The hormone from the 

 corpora lutea which inhibits ovulation during pregnancy also stimulates 

 the development of the mammary glands. Thus normal pregnancy is de- 

 pendent upon many balanced endocrine factors ranging all the way from 

 the periodic preparation for it to the insurance of a food supply in the 

 form of milk at its conclusion. A hormone which appears very early in 

 pregnancy makes it possible to diagnose human pregnancy with a high 

 degree of accuracy within the first month. 



The great importance of endocrine glands in controlling the later de- 

 velopment of vertebrates, particularly the role they may play in determin- 

 ing the conformations of various parts of the body, opens up the broad 

 question of internal secretions as factors in human development. There 

 can be no doubt that many physical and even mental abnormalities in man 

 are traceable to deficiencies of the endocrine glands or to upsets of their 

 normal interrelations. Atrophy or hypertrophy of such a gland may pro- 

 duce profound effects in the furthermost reaches of the body. Height, 

 broad or slender form, length of arms and legs, shape of face, quality of 

 voice, distribution of hair or of fat on body, and even emotions are in 

 greater or less measure conditioned by the relative functionings of the 

 various endocrine glands during earlier development and later life. And 

 there is no reason to doubt that the amount and quality of the secretions 

 in various family strains are as much the expression of hereditary factors 

 as many other individual characteristics. The hereditary aspects of these 

 glands, however, are likely to be overlooked, because they are also subject 

 to environmental modifications, and because we are accustomed to think 

 of them in terms of their immediate activities instead of their genetical 

 constitutions. 



Certain types of human defectives, such as cretins and so-called Mon- 

 goloids, even when of different races, often show marked resemblances. 

 The abnormalities in the case of cretins are ascribed to hormonal imbal- 

 ance — particularly to thyroid deficiency in the affected individual — and 

 those of the Mongoloids are suspected of being the result of endocrine 

 disturbances in the mother, or due to fetal nutritive insufficiency. 



Either thyroid or pituitary deficiency is an important factor in dwarfing, 

 but it should not be overlooked that inheritance may have been the de- 

 termining cause for the changed condition of the gland in the first place. 

 That such developmental anomalies cannot always be attributed to im- 

 proper functioning of an endocrine gland of the affected individual itself, 

 however, is shown by the fact that some such defects appear far back in 

 the early fetus before its endocrine glands are functional. Nor can the 

 abnormahty be attributed in all cases to endocrine defects of the mother, 



