EFFECTS OF VARIOUS EXTRANEOUS FACTORS 141 



differential substances are given off into the circulation. Blood vessels re- 

 spond to changes in the environment in certain respects similarly to the con- 

 nective tissue. With the latter, they move into necrotic tissue or into foreign 

 bodies soft enough to permit penetration by capillaries. But while the con- 

 nective tissue is stimulated more by homoiogenous than by autogenous dif- 

 ferentials, the blood capillaries are inhibited by the former and attracted 

 more actively by the latter. On the other hand, contact with autogenous tissue 

 tends to prevent the change of cellular connective tissue into fibrous tissue. 



However, tissue reactions can occur also between adjoining tissues of autog- 

 enous constitution and these reactions may be altered if a homoiogenous, 

 takes the place of the autogenous differential. Thus we have referred to the 

 changes which are seen when pigmented skin is autotransplanted into de- 

 fects in white skin in the guinea pig, or conversely, if white skin is trans- 

 planted into a defect in pigmented skin; the pigmented epidermis grows 

 into the unpigmented epidermis and this process continues for a cer- 

 tain time until new boundaries are produced between these tissues. In the 

 normal quiescent state of the tissues, each adjoins the other without re- 

 action; but whenever a tissue disturbance takes place, such as is caused 

 by the injury connected with transplantation, a struggle ensues between 

 the two adjacent types of epithelium. The pigmented epidermis is the stronger 

 one, but under normal conditions its superiority is merely potential ; it be- 

 comes activated under certain conditions which disturb the tissue equilib- 

 rium, and then a reaction occurs against tissue elements possessing the same 

 individuality differentials. If a syngenesio- or a homoiogenous differential 

 takes the place of the autogenous differential, this reaction is suppressed 

 The strange individuality differentials injure the tissue metabolism, as indi- 

 cated by the loss of pigment, which may occur in homoiogenous or syngenesi- 

 ous pigmented epidermis. The degree of inferiority of the unpigmented 

 epidermis and the mode of reaction of the tissues towards each other, if dis- 

 turbances take place, may vary in different species, even in nearly related 

 ones. In some instances, changes in the tissue equilibrium between pigmented 

 and white skin in the guinea pig may apparently arise spontaneously in non- 

 transplanted skin. Such an effect was observed by Saxton, Schmeckebier and 

 Kelley, presumably under conditions in which a disturbance of the tissue 

 equilibrium was due to some hidden metabolic change. 



As stated, the reactions above described were found in the guinea pig. 

 In the mouse, the transplanted pigmented skin does not extend into the 

 adjacent tissue, probably because the chromatophores here are not epidermal. 

 Also, in the tadpole conditions are, in certain respects, different. Thus Rand 

 and Pierce noted that while white transplants of ventral tadpole skin to pig- 

 mented dorsal skin were invaded by the adjoining pigmented host epidermis, 

 this could occur in autogenous as well as in homoiogenous transplants, yet, 

 an individuality differential was also involved in this reaction, as is indicated 

 by the fact that in many instances in autogenous grafts the desequilibration 

 was not sufficient to cause an invasion. An injury by homoiotoxins may then 

 have to be added, in order to overcome the inertia of the pigmented epidermis 



