THE TWO GREAT GROUPS OF CONNECTIVE-TISSUE CELLS. 43 



this tendency, doubtless, which has been mainly responsible for the confusion of the 

 fibroblastic segregation-apparatus, often filiform in character, with the mitochondria. 

 In the case of the fibroblasts it is in fact always possible to trace every transition 

 between small, globular vacuoles or granules and elongation of them into bacillus- 

 like, rod-like, or filiform structures. These forms must not be confused with the 

 crystallization process, which also gives delicate rod bodies and which can occur 

 equally well in the macrophage cells. A filar form of the segregation-apparatus in 

 macrophages, however, never occurs. This is hence a distinctive reaction displayed 

 by the fibroblastic cells. The filar structures may under certain circumstances 

 attain an altogether remarkable exaggeration, producing complex entanglements 

 of such "threads." These might legitimately be taken as hypertrophied mito- 

 chondria were criteria for distinguishing them not at hand (fig. 86). 



Before discussing the affinities of these structures, it would be well for us to 

 comment on the extent and characteristics of such "thread bodies" under various 

 experimental conditions. They are never displayed by the fibroblastic proto- 

 plasm of the mouse to the same extent as in the rat; and in the latter animal they 

 are typically produced only by certain dye substances and in particular areas. The 

 "thread bodies" are typically produced by trypan blue and its biological homo- 

 logues (figs. 1 1 and 87) . While the dyes which greatly enlarge the segregation-apparatus 

 of the fibroblasts (dyes 11, 228, and T 148) do not usually produce filar modifications of 

 the same, they nevertheless do accomplish this to some slight degree (fig. 82), and one 

 may see among the usual vesicular structures pointed prolongations of them produc- 

 ing comet or comma forms, or, again, rather sturdy long "threads." Even these 

 dye substances, however, produce extensive " thread bodies" in areas where they have 

 opportunity to affect the fibroblasts in a particularly intense way, e. g., in the areolar 

 tissue next the tunica vaginalis of the scrotal sac. Here the fibroblastic segregation- 

 apparatus exhibits its maximum tendency to undergo filar modifications and great, 

 rounded-up, true fibroblastic cells, whose protoplasm is full of a confused tangle of 

 "threads" may result (figs. 7 and 9). We have never under any circumstances 

 seen these structures in the protoplasm of the macrophage cells, nor do they in our 

 experience ever characterize any other cells of the body (e. g., mesothelium, endo- 

 thelium, lymphocytes). We have come to regard them, consequently, as dis- 

 tinctive characteristics of the fibroblasts, though they are not invariably displayed 

 by the latter. The filar structures carry weak but readily appreciable amounts of 

 the vital azo stains, which, since they can be demonstrated to develop from the vesi- 

 cles of the segregation-apparatus, must be derived from that source. They react 

 like the segregation-apparatus to supravital tinction with neutral red and, indeed, a 

 considerable abundance of them, on account of their very weak color, may be over- 

 looked unless neutral red is applied to the cell. They are particularly prone to 

 develop with trypan blue and its biological homologues. Vacuolar enlargements 

 may occur at either end of them, or along their course. Their delicacy and 

 straightness may sometimes lead one to interpret them as crystalline products; 

 but that they are not crystals of the dyes which produce them can be shown con- 

 clusively by the employment of dyes whose crystals possess a somewhat different 



