622 THE PHYSIOLOGY OF REPRODUCTION 



essential factor in both mammary growth and the initiation of 

 mammary secretion, and no contradiction is involved in the assump- 

 tion that the hormone or hormones when gradually secreted into the 

 circulating blood in a state of nature act differently from the sudden 

 injection of a considerable quantity of the substances. It is possible 

 also that the hprmone which has the power of increasing muscular 

 contractility is produced in greater abundance at about the time 

 of parturition (see p. 577). 



It has been already shown that after ovariotomy lactation may 

 be prolonged for an almost indefinite period, and further, that after 

 the same operation the pituitary gland undergoes hypertrophy. It 

 is not unlikely that these two facts are correlated, and that after the 

 removal of the ovaries the function of the corpus luteum in relation 

 to milk secretion may be taken over vicariously by the pituitary, 

 which, unlike the luteal gland, does not atrophy after a comparatively 

 short period. Such a conclusion, however, is at the best a very 

 tentative one, for although the corpus luteum in Dasyurus l and the 

 rat z is said to persist during lactation, it is not known how long the 

 organ continues to function in other Mammals. 3 Relatively to 

 the duration of suckling the corpus luteum in most species probably 

 persists for only a short time. 



1 Sandes, " Tlie Corpus Luteum of Dcuyurus" Proc. Linn. Soc. N.S. W., 

 vol. xxviii., 1903. 



2 Watson (B. P.), "On the State of the Ovaries during Lactation," Proc. 

 Phi/s. 8oc., Jour, of Physiol., vol. xxxiv., 1906. 



3 The part of the pituitary which hypertrophies after ovariotomy is said to 

 be the anterior lobe, while the part that elaborates the galactogogue is the 

 posterior lobe. The hypothesis suggested above, therefore, assumes a functional 

 relation between the two lobes of which there is some evidence. 



