48 DEVELOPMENT OF CONNECTIVE-TISSUE FIBERS. 



entiated structure loses its characteristic appearance. Also, the cells of the new 

 growth have a tendency to migrate away as individual cells instead of developing 

 into a differentiated tissue composed of numerous cells. In all probability the 

 cells do not de-differentiate and become more embryonic, as has been claimed by 

 Champy (1913) and others, but simply lose their characteristic differentiated 

 appearance, due to their changed shape and position. This is interestingly shown by 

 a study of smooth muscle-cells (plate 2, rig. 6). Where the cells are attached closely 

 to the cover-slip they no longer contract and the myofibrils appear as irregular 

 bundles composed of numbers of delicate fibrils (plate 2, fig. 6). However, where 

 the cell is not so closely attached to the cover-slip it continues to contract, and in 

 this case the myofibrils are arranged into the characteristic fibrils. Taking the 

 possible loss of the characteristic appearance of the differentiated structure into 

 account, the very thin and largely spread-out living cells of the tissue culture furnish 

 an excellent means for the study from day to day of certain structures of the cell. 



Just how much differentiation can take place in such cells in these tissue 

 cultures is difficult to state. Certainly in a few cases, where the mesenchyme 

 growth at 48 hours was composed of quite undifferentiated cells, this growth, 

 when kept alive by frequent baths of fresh solution, did develop definite connec- 

 tive-tissue fibrils. Muscle fibers have been observed to become more differen- 

 tiated; but in the case both of the muscle fibers and connective tissue there is 

 some continuation of function, as the muscle fibers frequently contract, and the 

 connective-tissue growth also occasionally contracts back around the explanted 

 piece and later grows out again. 



Fibrils did not develop in many of the cultures of connective tissue, owing 

 to the fact that the cells remained spread out as individual cells until the death 

 of the culture. In the few cultures that were kept alive for a sufficient length of 

 time, and in which the connective-tissue fibers did develop, they could be clearly 

 seen and studied in the living preparation from day to day, and their develop- 

 ment could be traced from the earliest delicate fibril within the exoplasm of the 

 cell to the more adult fibers, which appear to be free from the cells. 



PREVIOUS WRITINGS ON THE LIVING CONNECTIVE TISSUE. 



Whether the connective-tissue fibers arise within the cells or from an inter- 

 cellular substance is still an open question. The weight of evidence seems to be 

 in favor of a cellular origin, although certain text-books of histology present the 

 question almost whoUy from the intercellular point of view. 



There are many reviews of the literature on both sides of the question 

 (Fleming, 1891; Spuler, 1896; Mall, 1901; Rothig, 1907), and also various text- 

 books, and since the technique used by other investigators is so different from 

 that employed in the following observations no effort will be made to take up in 

 detail the various papers upon the origin of the connective-tissue fibers. 



While many observers have studied teased preparations of connective tissue, 

 Boll (1872) was the first to study the development of the connective-tissue fibrils 

 entirely from the living cell. Boll made his cultures by teasing out a few of the 



