612 THE PHYSIOLOGY OF REPRODUCTION 



reached in a rabbit about eight days pregnant. In another rabbit 

 which received twenty-four injections, spread over five and a half 

 weeks, the effects produced were still more marked. Further experi- 

 ments showed that boiled extract was as effective as unboiled, and 

 the conclusion was therefore drawn that in all probability the specific 

 secretion or hormone is capable of withstanding boiling. It was 

 shown also that the substance in question could be obtained equally 

 well from different parts of the foetus, that it passes through a 

 Berkefeld filter, and that it is not retained to any appreciable 

 extent by the kieselguhr in Buchner's method for extracting cell 

 juices. 



Foa states that extract of ox fostus, when injected into rabbits, 

 produced development of the mammary glands. He concluded, 

 therefore, that the stimulating substance which causes mammary 

 growth is not specific i.e. not peculiar to any one kind of mammal. 

 Foa says also that if the extract is heated to 1 10 the active substance 

 is destroyed, and no result is produced by injection. 1 



Since the growth of the mammary glands was apparently due to 

 a specific chemical stimulus arising in the foetus, it was natural to 

 suppose that the beginning of the actual secretory process which 

 marks the cessation of growth was caused by the removal of this 

 stimulus in other words, by the expulsion of the fretus. In this 

 connection it is interesting to note that abortion or premature 

 labour is often followed by the appearance of milk in the 

 breasts. 



The idea that lactation is due to the removal of an inhibition 

 appears to have been entertained first by Hildebrandt,- who put 

 forward the suggestion that the developing embryo exerts an 

 influence whereby the cells of the mammary gland are protected 

 from those autolytic disintegrative processes which are supposed to 

 occur during active secretion. That the act of secretion is to be 

 ascribed to autolytic processes of the gland, is, according to Miss 

 Lane-Claypon and Starling, highly improbable, and there is no 

 evidence that the autolysis of the gland-cells would give rise to the 

 specific constituents which characterise milk. 



Halban 3 has put forward the view that the specific stimulus for 

 niiiiiimary development arises in the placenta, while the active 

 secretion of the mammary glands is determined by the expulsion or 



"Sui Fattori che determinant) 1'Accrescimento e la Funzione della 

 (Miiandola Mamrnaria," J /</,. ,// /'/*., vol. v., 1908. 



'-' Hildebrandt, " Die Lehre von der Milchbildung," Uofmeistei's Heitr>i<f'\ 

 vol. v., 1904. 



; llalban, loc. cit. See also Bouchacourt, C. R. dc la Soc. de Biol., 1902; 

 and Lederer and Przibram, PflAgefs Arch., vol. cxliv., 1910. The latter authors 

 found that extract of placenta injected into a goat caused a marked increase in 

 milk, while ovarian extract had no effect. 



