CHEMICAL CORRELATION 563 



responsive organ. Such a state of increased functional activity 

 cannot be without significance for the nutrition of the tissues 

 concerned. We know that the most effective means of exciting 

 hypertrophy of any organ is to augment the calls upon this 

 organ — i.e. to give it increased work to do. We should expect 

 therefore that the indirect effect of these hormones or Reizstoffe 

 would be an improved nutrition, and possibly increased growth, 

 of the organs concerned. Another group of correlations exists 

 in which increased activity is but an indirect effect, the primary 

 result of the action of the hormone being diminution of activity, 

 accompanied by increased assimilation and hypertrophy of the 

 tissue. 



The most striking instances in which growth is the primary 

 effect of a chemical stimulus derived from some distinct organ 

 are to be found in the correlations existing between the genera- 

 tive organs and the rest of the body. Although the manner 

 in which this correlation is brought about has been the subject 

 of speculation for many years, it is only quite recently that 

 any attempt has been made to apply experimental methods 

 to its explanation. 



Especially interesting is the mechanism by which growth 

 is aroused in the mammary glands. These organs are present 

 in both sexes at birth in an immature condition. At puberty 

 for the first time a difference appears between the mammary 

 glands of the two sexes, a rapid growth taking place in the 

 female simultaneously with the commencement of the ovarian 

 functions. During the whole of sexual life the glands remain in 

 the female at the same stage of development, unless pregnancy 

 occurs. The onset of pregnancy acts as an impetus to a further 

 great development of the gland substance, which continues 

 with ever-increasing rapidity throughout the whole of preg- 

 nancy. At parturition the growth of the glands at once ceases, 

 and one to three days later we find that the activity previously 

 spent on growth is now applied to the secretion of milk — a 

 secretion which, if the gland be emptied periodically, may last 

 many months. 



Since the whole cycle of changes can be prevented by 

 removal of the ovaries, we must regard these organs as 

 primarily responsible for the growth of the mammary glands, 

 though whether they are the direct source of the impulses 

 which determine the special growth during pregnancy, or 



