CELLS LINING THE PERITONEAL CAVITY A 
but they have done even more in giving us a very large number 
of figures of fibroblasts stained with various dyes. In studying 
their plates it becomes evident that there is no pattern which 
one can consider as in common between even a small number of 
the fibroblasts. 
The typical fibroblast therefore shows no characteristic stain- 
ing in a definite area of the cytoplasm such as is so evident in 
the serosal cells from the entire peritoneal membrane. Indeed, 
the reactions of the fibroblast to vital dyes would seem from 
these plates alone to ally them more closely to the clasmatocytes 
than to the serosal cells. Many observers have described reac- 
tions of connective-tissue cells and serosal cells as being quite 
similar during the different stages of inflammations, and on such 
grounds as these have considered that they are interchangeable 
(Cornil, ’97; Ranvier, ’93; Schott, 09; Weidenreich, ’07; Roloff, 
94, 96, and Dominici, ’01, ’02). 
Clarke (16), who introduced celloidin and paraffin into the 
general connective tissues, considered that he had produced a 
true experimental mesothelium which, according to him, was 
entirely comparable to the lining cells of the peritoneum. Among 
the older workers many thought that the mesothelial cells were 
capable of transformation into a ‘variety of forms,’ one of which 
was the fibroblast. Schott (09) and Weidenreich (’07) suggest 
that the surface-cells of the omentum are merely flattened fibro- 
blasts. Mallory (20), in studying tumors of the arachnoid, has 
concluded that they are of connective-tissue character, and 
because of these tumors arising from the arachnoidal cells, 
he concluded that these cells must be considered as flattened 
fibroblasts. 
The work done on the differentiation of fibroblasts into serosal 
cells seems to me inconclusive. That the normal serosal 
lining cell has a different reaction to vital dyes from that mani- 
fested by the fibroblast is undoubted, but no effort has yet been 
made to determine whether the cells which have formed around 
foreign bodies or those which have recovered denuded areas of 
peritoneum give reactions to vital dyes similar to those reported 
here for serosal cells. It seems, therefore, that the evidence so 
