DIGESTIVE ACTIVITY OF MESENCHYME 447 



their growth is seldom found to be more than a narrow strand, 

 and usually both tissues come together on the fourth to the sixth 

 day after grafting. There is little to add to the description of the 

 inauguration of tumor growth given in the preceding section; 

 the proliferation of the tissue proceeds in the proximity of an 

 adult spleen graft in the first days after grafting in the same way 

 as does the single tumor graft. 



An inhibition of the growth of the tumor graft usually does 

 not begin until the two grafts have come together; but in cases 

 in which a close contact between the tissues is not effected, the 

 growth of the tumor may still be inhibited at a later period even 

 before the prehatching changes in the allantoic circulation have 

 begun. The inhibition in this case depends upon general changes 

 in the host's mesenchyme called forth by the presence of a" 

 growing adult spleen graft on the allantois. The factors of the 

 growth restraint over a tumor grafted in close touch with an 

 adult spleen graft can, therefore, be distinguished as local or 

 direct and general or secondary. Growth restraint is locally 

 brought about by the activity of the groups of grafted splenic 

 mesenchymal cells that come into contact with tumor cells. 

 The general or secondary factors concerned in the inhibition of 

 the growth of the tumor consist in the general changes produced 

 by a spleen graft upon the mesenchyme of the host, of which the 

 allantois is a part. These changes consist in an ubiquitous 

 myeloid metaplasia, as I have described in several papers. In 

 an adjacent double graft the local and general factors of growth 

 restraint are at work together, and it is difficult to distinguish 

 their separate spheres of action. The unfavorable effect of the 

 myeloid metaplasia of the host's mesenchyme on tumor growth 

 can, however, be distinguished from the direct damaging activity 

 of the splenic mesenchyme and studied separately in grafts of 

 spleen and tumor placed at a distance of about 2 to 3 cm. from 

 each other. In this case the much milder inhibition of the 

 tumor growth, though easily noticeable if the myeloid metaplasia 

 is intense, depends exclusively upon the changes in the host's 

 mesenchyme. These secondary factors of inhibition of tumor 

 growth will be analyzed beyond; only the local factors depend- 



