52 THE AMERICAN NATURALIST [Vol. LIV 



Correspondingly we find least dense fibrous tissue 

 around actively metabolizing parenchyma, while around 

 the metabolically less active excretory ducts there is 

 usually a coat of much more dense fibrous tissue. Thus 

 it might be tentatively suggested that a decrease in these 

 autosubstances, to the existence of which the results of 

 tissue transplantation point, is at least partially respon- 

 sible for the changes which take place in old age. Other 

 factors of a different nature play certainly a part. Thus 

 the corpus luteum stimulates the activity of the uterine 

 connective tissue and epithelium. In later periods of 

 life this stimulating influence is lacking. 



While the autosubstances increase directly or indi- 

 rectly the vascular supply, they very effectively limit the 

 growth of fibroblasts in contact with epithelial elements. 

 We have previously shown through transplantation ex- 

 periments that during regeneration the epithelium limits 

 the growth of the connective tissue. 2 In a similar way 

 perhaps the endothelium of the blood vessels limits the 

 growth of the connective tissue. Our further experi- 

 ments lead to the conclusion thai this restraining influ- 

 ence exerted by endothelium and epithelium upon con- 

 nective tissue migration and proliferation is a specific 

 effect. The auto-substances possess it in the highest de- 

 gree. It is less marked in the case of syngenesio- and 

 still less in the case of homoiotoxins. But even the 

 hetero-substances still exert a certain restraining influ- 

 ence, at least temporarily on the connective tissue and 

 they are thus in this respect more effective than foreign 

 bodies. It is again clear that considering the tendency 

 of fibroblasts and lymphocytes to migrate actively into 

 parenchyma and to destroy it, the epithelial structures 

 can be assured of preservation only, if the auto-sub- 

 stances possess some kind of a restraining influence on 

 the connective tissue. 



It is probable that not only in old age but also under 

 certain pathological conditions, the activity of these auto- 



