191 



culture of pure fibroblasts which liad not been divided in 

 the same number of generations. At this stage, the 'nixed 

 cultures were divided and subcultured. This procediu'e was 

 continued for seven passages and finally the preparation 

 was fixed, sectioned and stained b}' Van G i e s o n's metliod. 

 The sections sliowed typical epitlielial and connective tis- 

 sues, as found in the organism. The epithelium appeared 

 greenish yellow in contrast with the connective tissue, wliich 

 appeared pink and contained many fibrillae which were 

 stained a decided pink. No parts of the section 

 s h o \\' e d a n a m alga m a t i o n of the t \\- o cell t }- p e s, 

 figs 20. 21; The epithelial cells were everywhere dislinclly 

 differentiated. In many places a definite structural arrange- 

 ment of the cell elements was observed. The epithelial cells 

 had grouped themselves to form tubules with distinct lumina. 

 In several of the sections, the lumina could be seen filled 

 with a homogeneous colloidal secretion. The arrangement of 

 the epithelial cells forming the tubules resembled the con- 

 formation found in sections of salivary glands. The individual 

 cells which formed the tubules had their nuclei disposed 

 close to the basal membrane. 



In some parts of the section, epithelial cells could be 

 seen penetrating the surrounding layer of fibroblasts and ap- 

 pearing on the free surface of the tissue fragment. (Fig. 21.) 

 In other parts, large masses of keratinized cells, surrounded 

 by a layer of low epithelium, could be observed. 



These experiments show that epithelium cultivated for 

 2 months in vitro retained its morphological characteristics 

 which differed decidedly from those of fibroblasts. But a 

 still more striking fact was observed; namely, that the dif- 

 ferential stain ad modum Van G i e s o n brings out the 

 chemical difference between the two cell ts-pes w^hen they 

 are allowed to grow^ together. The epithelium was observed 

 to have formative ability; i. e., the epithelial cells arrange 

 themselves in winding tubules. This has already been men- 

 tioned in an earlier report of experiment in which the 



