THE TWO GREAT GROUPS OF CONNECTIVE-TISSUE CELLS. 47 



fibroblasts, a fact familiar to us in the production of granulation tissue and evi- 

 denced in the most spectacular way by their exuberant growth in tissue cultures. 



This is not the place for a discussion of the interesting question of the affinities 

 of either of the areolar cell types with mesothelium, but the single statement may 

 be made that azo dyestuffs have very distinctly helped our power of discrimination 

 and separation of these cells. In the specificity of this cell type, which is cham- 

 pioned by Tscahschin against the views of Ranvier, Schott, and Weidenreich, we 

 find ourselves in hearty agreement. The normal mesothelium is even freer from 

 vacuolar bodies than are the fibroblasts and hence has closer affinities with the 

 fibroblasts than with the macrophage cells. 1 Its cells can always be distinguished 

 from both of the connective-tissue elements by means of the vital azo dyes. Such a 

 distinction, it is again to be emphasized, is not made on the basis of a unique reaction 

 to the dye different in kind from that displayed by the connective-tissue cells, but by 

 the fact that with any particular dosage and dye the form, size, distribution, and 

 abundance of the mesothelial segregation-apparatus thus called into existence to house 

 the vital dye will assume characteristics no less peculiar for the mesothelial than 

 for either of the connective-tissue cells. Mesothelial cell deposits are usually 

 minute ones. Even with protracted intraperitoneal dosage, they do not, in our ex- 

 perience, show the filar structures occasionally provoked in the fibroblasts, nor do 

 they acquire a vacuolar apparatus as great as that possessed by the subperitoneal 

 contiguous macrophage cells, even in cases where they are completely filled by dye 

 storage, as is frequently true of the mesothelium of the spleen. 



The mesothelial cells can also become the seat of true crystalline deposits of 

 the vital dye either as a result of great dosage (our saturation crystals) or by the 

 method of prolonged low dosage with some of the crystallizable members of this 

 extensive series of dyes. Claims for the participation of mesothelium in true fibro- 

 blast production would not, from any theoretical deductions, be wisely denied, but 

 this will have to be investigated with the aid of specific cytological criteria fur- 

 nished by an agent or agents at least as satisfactory as the vital azo dyes. The 

 methods hitherto employed have not been adequate for such a task. As far as 

 the occurrence of macrophage production from the mesothelium is concerned, 

 it will be recalled that this has indeed been taken by many observers as the source 

 of the free cells of the peritoneal cavity. A renewed study of this question with better 

 methods of discrimination would likewise be indicated, and more especially since 

 so good an authority as Weidenreich has been willing to derive the macrophages of 

 the peritoneal exudate from almost every possible source in the mesothelial, the 

 wandering, the hemotogenous, or the connective-tissue cells. If the claims for the 

 participation of the mesothelium in macrophage or fibroblast production are, then, 

 perhaps not justly stigmatized as groundless, it yet appears certain from studies 

 with the benzidine dyes that mesothelium itself is produced only from mesothelium. 



1 Renaut has also been struck with the absence of a "rhagiocrine function" in the normal definitive pleuro-peritoneal 

 endothelial cells. These elements, however, receive azo-dye deposits which are not less characteristic than those of the other 

 vitally stainable cells. 





