THE HORMONES IN PREGNANCY 



Through this pool the mother's blood slowly flows, out from 

 the arteries that supply the region and back into the veins, 

 bathing the rootlike villi of the placenta. From the mother's 

 blood to the blood of the embryo, oxygen and dissolved food- 

 stuffs filter through the covering cells of the villi and through 

 the thin walls of the blood vessels that run along within them, 

 just as nutritive substances pass into the roots of a plant 

 from the moist soil in which they grow. The infant sends back 

 carbon dioxide, urea, and other wastes from its tissues, to be 

 filtered out through the placental vessels into the mother's 

 blood, which carries away these wastes to be disposed of with 

 the products of her own metabolism. 



The covering cells of the villi are called upon, however, not 

 only to take part in the process of filtering foodstuffs inward 

 and waste products out, but also to produce a whole series of 

 endocrine substances. This important fact is unfortunately 

 masked by a great deal of confusion in our present knowledge. 

 The placenta differs greatly in different species, not only in 

 its structure but also in its endocrine activities. Statements 

 which are true of one species may not apply at all in others. 

 For this reason our discussion will be limited to the human 

 species (except as specifically stated) and even then we shall 

 be restrained by a certain amount of uncertainty. To begin 

 with, the placenta begins very early in pregnancy to elaborate 

 a hormone of its own, having profound gonadotrophic prop- 

 erties (that is, power to stimulate activity of the ovary and 

 the testes) much like that of the pituitary gonadotrophic 

 hormones.^ This activity begins, indeed, before we can prop- 

 erly speak of the placenta, for the hormone in question is 

 made by the cells (i.e. the trophoblast) covering the early 

 villous processes that surround the embryo, as soon as they 



1 For the sake of clearness, it seems best to refer to the gonadotrophic 

 material in the singular, i.e. "a hormone," but actually it seems to be a 

 hormone complex comprising two substances, one of which tends to 

 stimulate growth of the follicles, the other to convert the follicles into 

 corpora lutea (Appendix II, note 11). 



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