THE HORMONES IN HUMAN REPRODUCTION 



dition, by action of the estrogenic hormone. Nothing is more 

 striking than to watch the growth of the mammary glands in 

 a young animal receiving estrogen. Each gland is a system of 

 branching channels lined by cells derived from the outer layer 

 of the skin (epidermis). Long before birth these ducts begin 

 to grow from the nipples and to spread out around them in 

 a little circle under the skin. At first the channels are few and 

 short, and only slightly branched (Fig. 29, A). Under the 

 action of the estrogenic hormone they branch extensively and 

 spread to adult dimensions (as indicated in the diagram, Fig. 

 29, B). In the girl at puberty this is, of course, a gradual 

 change, but in an experimental animal under hormone treat- 

 ment it can be produced quite rapidly. The nipples quadruple 

 in size in a few days, and the ducts push outward in a widen- 

 ing circle. In pregnancy a much greater development occurs. 

 The branches of the duct system develop extensive terminal 

 twigs ending in secretory alveoli (Fig. 29, C, B). These be- 

 come more numerous as pregnancy advances. Finally, globules 

 of milk-fat accumulate in the cells of the alveoli (Fig. 29, E). 

 The actual flow of milk in quantity does not begin, however, 

 until after parturition. 



From the time it was first conjectured that the corpus 

 luteum is a gland of internal secretion, until quite recently, it 

 was supposed that this particular endocrine organ is respon- 

 sible for the growth of the mammary gland in pregnancy and 

 the secretion of milk. On the face of it, there could hardly be 

 a more plausible conjecture, for the growth of the mammary 

 gland closely follows the appearance of the corpus luteum, 

 and is so obviously a part of the general preparation for the 

 infant that it seems very logically to go with the other func- 

 tions that the ovary exerts during pregnancy. 



If this is true, however, why does not the corpus luteum 

 produce mammary growth and even lactation not only in 

 pregnancy but in each ovarian cycle.? To this query A. S. 

 Parkes of London in 1929 offered the tentative reply that 



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