195 



In my experience, Ihe keratinization in epiUielial cul- 

 tures, is always the result of the culture suffering from lack 

 of oxygen and nourishment. It cannot be denied, however, 

 that the presence of fibroblasts, as Drew has described it, 

 can give the same result, but it is only to be demonstrated 

 here, that both phenomena occur generally without the 

 presence of fibroblasts, and that the orginisaliou according 

 to D r e w, the "differentiating" factor of connective tissue, 

 is not a specific reaction of these cells on epithelium. It is 

 nevertheless an interesting fact, that connective tissue cells, 

 cultivated side by side with epithelium, bring about a quick 

 organisation and keratinization of the cells; it demonstrates 

 probably only, that the intrinsic power of fibroblasts is much 

 greater, compared with that of the epithelium. 



It is worth while mentioning here the fact that I he 

 epithelium used for the cultures is iris epithelium, which 

 does not generally keratinize in the organism. 



In order to get a suitable subject for the study of the 

 interdependancy of the various tissues in vitro, a series of ex- 

 periments was undertaken with the cultivation of complex 

 tissues, i. e. tissues consisting of several types of cells. 



If a part of an organ or a complete embryo was oxplanted, 

 it increased markedly in size and no outgrowth of cells took 

 place. Thomson ^^^) found that if a toe of a chicken 

 embryo, for instance, was explanted, an uncontrolled pro- 

 liferation of cells began from the injured part. When the 

 basal membranes are injured, the cells begin to grow out 

 into the medium, whereas an uninjured organ continues to 

 grow as a complete structure controlled by the laws of the 

 organism. 



This statement of Thomson could not be quite con- 

 fiiTned by my experiments. By explanting small bits of 

 intestines from chicken embryos I observed very seldom 

 any uncontrolled growth; the basal membranes of the tissues 

 in the intestine must have been injured by the excision. 



13* 



