274 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



malian organism, also, some special substances are given off by tissues which 

 influence the state of adjoining tissues. They may be contact substances, 

 comparable in certain respects to the organizers of embryonal tissues. Thus, 

 the egg in the ovary may stimulate the growth of the surrounding follicular 

 granulosa and the state of the parenchyma may change the condition of the 

 surrounding stroma; but also, the blood vessels and their permeability may 

 affect the stroma in which they are embedded, and by way of the stroma they 

 may affect even the parenchyma. Local defects may alter the tissue equilibrium, 

 inducing tissue growth, and even without such defects neighboring pigmented 

 epidermis may, under certain conditions, invade unpigmented epidermis. In a 

 similar maner the squamous epithelium of the cervix, which develops under 

 the influence of hormones, may act towards the neighboring cylindrical epi- 

 thelium of the uterus. These exists, in all probability, other local mechanisms 

 which maintain the tissue equilibrium in addition to the action of hormones 

 originating in distant places. We may then conclude that the normal tissue 

 equilibrium depends (1) upon the action of autogenous differentials, which 

 all tissues possess, and (2) upon a variety of other effects, among which the 

 action of some special hormone-like contact substances as well as typical 

 hormones play a prominent role. There is thus a certain correspondence be- 

 tween the factors which determine the interaction of embryonal tissues and 

 those which determine the autogenous equilibrium of the adult higher or- 

 ganisms. 



