I6O LEO LOEB. 



lymphocytes in the most adequate way, and in a manner peculiar 

 to each organ, in a way which keeps away lymphocytes, which 

 limits the activity of fibroblasts which under those conditions 

 cannot invade the tissue of another kind and merely enter in such 

 relations with these tissues as are demanded by the normal struc- 

 ture and function of the organ. The auto substances thus bring 

 about the restitution of the normal structure in an at first disor- 

 ganized organ, just as these substances maintain the normal struc- 

 ture and function in the normal organ. The syngenesio, homoio 

 and hetero conditions on the other hand lead to those pathological 

 conditions which we have described. There are other facts which 

 equally point to the conclusion that substances w r hich in general 

 we may designate as " contact substances " are given off and 

 regulate the relations of various kinds of tissue to each other. 



We may mention a few examples of conditions under which 

 contact substances come into play. 



(a) In our study of the cyclic changes in the mammary gland 

 we found a relation between the state of the glandular paren- 

 chyma and the surrounding connective tissue stroma. We found 

 that an active gland has an active stroma, while a resting, retro- 

 gressing gland has a resting, more or less fibrous stroma. 



(6) Similarly we find around the various gland ducts which 

 are metabolically inactive usually a resting fibrous stroma, in con- 

 tradistinction to the cellular stroma around the active gland tissue. 



i c ) Wherever cells of the parenchyma are multiplying we 

 generally find the stroma cells and the vessels to become like- 

 wise active and conversely where connective tissue cells and the 

 vessels are active the cells of the parenchyma, for instance, epi- 

 thelium, receive a stimulus to grow. 



(d) There are indications that contact substances play also 

 a certain role during embryonic life and that here they help to 

 determine the formation of some organs. Thus a lens producing 

 contact substance is given off by the optic disc. It stimulates 

 growth and a special differentiation in the overlying ectoderm. 



Thus we know at the present time a large class of contact sub- 

 stances and it is probable that future studies will still add to the 

 number of these substances. 1 



1 Several years ago Dr. Walsh and the writer found that a substance given 

 off by the ovum determines the development of the follicle in the ovary. 



