117 



by adding chicken-plasma to llic medium) the epithelial 

 cells grew out in big sheets of |)()lygonal cells. This is an 

 important lad and will |)robably explain a good many 

 things in the morphology of cells. The mechanism of these 

 changes in moi-phology, according to the structure of the 

 solid phases in the cnltui'e medium has been subjected to 

 several explanations. The medium which U h 1 e n h u t h ^^^) 

 calls soft is composed of a relatively small amount of solid 

 phase, i. e., the fibrin fibrillae are very sparsely represented. 

 In this medium the epithelial cells grow out on the sparse 

 fibrin threads and consequently become si)indle-shaped be- 

 cause of llie relatively big spaces of liquid in between 

 them. Wv know that the tissue cells must have a stroma 

 to grow on, otherwise they become I'ound and die. In the 

 hard media we have a dense meshwork of fibrin threads, 

 which acts more like one i)ig snlid phase; the cells are 

 able to spread themselves out as epithelium naturally does. 

 As the ei)ithelial cells have been cultivated in vitro in 

 between a liquid (tissue juice) and a solid ffibrin) phase, 

 the cells appear as they do in the organism under natural 

 conditions, namely, as a pavement in solid sheets. (Fig. 10.) 

 When fibroblasts are cultivated on the surface of the plasma- 

 clot, they immediately embed themselves in the medium. It 

 seems as if the ei)ithelial cells are inhibited by the mecha- 

 nical resistence of the fibrin-fibrillae to a much greater ex- 

 tent than are the connective tissue cells. When the epithelium 

 is cultivated embedded in the clot, the growth seems to 

 depend very much upon the existing condition within the 

 medium and the disposition of the fragment. If the condition 

 of the coagulum })revents the uniform outgrowth of new 

 cells from the mother fragment and when the condition of 

 respiration of the fragment is not of the very best, then the 

 cell-invasion is characterized by the formation of branching 

 tubules of various forms. (Fig. 15) but essentially the ar- 

 rangement of the growing cells is such as to form a more 

 or less organized structure resembling hollow tubules. The 



