516 THE PHYSIOLOGY OF REPRODUCTION 



of Hammett, 1 who found that the human placenta contains urea, and 

 that the urea content is greatly increased in this organ in toxsemic 

 pregnancies. He also found that urea is formed when the placenta is 

 allowed to undergo autolysis. No proof exists of the excretion of urea 

 otherwise than into the liquor anmii. Nor does its presence in the 

 amniotic fluid necessitate an oxidation of protein ; it may be split off, 

 as in the adult, by a simple hydrolytic cleavage. At present we must 

 be content with assuming the possibility of modifications in the 

 maternal blood from the presence of foetal nutritive and waste 

 materials. Hitherto the investigations have been largely confined to 

 human pregnancies, in which individual differences are at a maximum, 

 and the application of the experimental method is restricted. Hence 

 our knowledge of the chemical changes in the blood is very limited. 

 Its composition may, in addition, be modified by the activities of the 

 placenta itself. Several theories have been put forward in support 

 of the view that this organ acts as an internally secreting gland. 

 Nattan-Larrier 2 goes so far as to state that the secretion can be 

 demonstrated in the form of globules lying on the surface of the villi, 

 but these arise in the post-mortem degeneration of the tissue. Of the 

 same nature are the products of the placenta which have a blood- 

 pressure raising action. Extracts of the fresh organ have no pressor 

 effects, nor do they increase uterine contractions. 3 Halban 4 considers 

 that the placenta secretes a hormone which stimulates the growth of 

 the mammary gland and the secretion of milk. 5 Starling 6 suggested, 

 on the other hand, that the hormone was contained in the tissues of 

 the foetus, that by its activity during pregnancy it led to a prolifera- 

 tion of the mammary tissue, and that the cessation of the stimulus 

 after parturition brought on the secretion of milk. 



According to Liepmann, 7 the maternal blood contains a special 

 protein, elaborated by the placenta, which may be recognised by the 



1 Hammett, "The Urea Content of Placentas from Normal and Toxsemic 

 Pregnancies," Jour. Biol. Chem., vol. xxxiv., 1918 ; " Urea Formation by the 

 Placenta," ibid., vol. iii., 1919. 



2 Nattan-Larrier, "Fonction Secretoire du Placenta,," Compt. Rend. Soc. 

 Biol., vol. Hi., 1900. 



3 See footnote 6 , p. 557. 



4 Halban, " Die innere Secretion von Ovarium und Placenta, und ihre 

 Bedeutung fur die Function der Milchdriise," Arch. f. Gyndk., vol. Ixxv., 1905. 



5 In future investigations, the better recognition of the composite structure 

 of the placenta is desirable. In many animals it is possible to separate the 

 maternal and foetal tissues with considerable accuracy, and any effect obtained 

 from one or other part can be definitely ascribed to the modified uterine 

 mucous membrane, or to the extra-embryonic part of the ovum. 



6 Starling, " Chemical Correlations of the Functions of the Body," Croonian 

 Lesjures, Lancet, 1905. Starling appears, however, to have abandoned this 

 theory in view of the more recent work on the functions of the corpus luteum 

 (see p. 616). ' 



7 Liepmann, " Ueber ein fiir menschliche Plazenta spezifisches Serum," 

 Deut. med. Woch., 1902, 1903. 



