492 THE PHYSIOLOGY OF REPRODUCTION 



results entirely from the combustion of carbohydrates in the 

 mammalian foetus, is excreted into the maternal circulation 

 through the placenta. With regard to a wastage in the protein 

 metabolism, a certain loss is bound to occur in the transforma- 

 tion of " circulating " or " fixed " maternal proteins into foetal 

 tissue proteins ; and in addition, incompletely oxidised substances 

 may possibly be transmitted to the placenta and oxidised there 

 or in the mother (Bohr 1 ). The question of urea formation by 

 the foetal liver or the trophoblast is still uninvestigated, and 

 no proof exists of the excretion of urea otherwise than into the 

 liquor amnii. 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 con- 

 fined to human pregnancies, in which individual differences are 

 at a maximum, and the application ol 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 mam- 

 mary gland and the secretion of milk. Starling 5 states, on the 



1 Bohr, see Nagel's Handbuch der Physiologic, " Respiration," vol. i., H.i. 



2 Nattan-Larrier, "Fonction Secretaire du Placenta," Comp. Rend. Soc. 

 BioL, vol. lii., 1900. 3 See foot-note 2 , p. 522, 



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

 Bedeuturig fur die Function der Milchdriise," Arch. f. Gyntik., vol. Ixxv., 

 1905. 



5 Starling, "Chemical Correlations of the Functions of the Body," 

 Croonian Lect., Lancet, 1905. 



