54 DEVELOPMENT OF CONNECTIVE-TISSUE FIBERS. 



it becomes stellate in form and later divides by mitosis and may again develop 

 long and delicate processes among the cells of the new growth. 



From a study of this cell (plate 1, fig. 3) and of the cells shown in plate 1, figure 8, 

 and plate 2, figure 4, it can be seen that in the embryo, where the growth is in all direc- 

 tions rather than in a flat plane (as in tissue cultures), a section must necessarily cut 

 many of these delicate processes and cause the appearance of a network of isolated 

 protoplasmic threads between the cells, because the connection of these threads 

 with the cells to which they belong is not shown in the section. 



Typical spindle-shaped cells never appeared in pure cultures of connective 

 tissue, but always in those which contained muscle-cells; and in every instance a 

 typical spindle-cell could be identified as a muscle-cell. 



In certain of the explanted pieces spindle-cells were observed, but these 

 appeared to be due to the pull which had been put upon the tissue during manip- 

 ulation, for frequently parallel bands of fibers extended along these cells. 



In some preparations from a 10-day chick embryo the cells were connected 

 by so many delicate processes that a network of these processes was formed 

 between them, which would have been difficult to identify as cellular in origin 

 had it not been for the fact that during the mitosis of one of these cells all the 

 delicate network connected with the cell was partly drawn into it, and the space 

 around the cell became free from network (plate 2, fig. 5) . It thus became clear that 

 the protoplasmic network between these cells was not extracellular in origin. 



The fibrils appeared first (after 24 hours' growth) as slightly more refractive 

 lines within the cytoplasm of the individual cells (plate 2, figs. 1 and 9) . The mito- 

 chondria were frequently stretched along these delicate lines; by careful study, 

 however, it was seen that the mitochondria did not take part in the formation 

 of the cellular fibrils, but that even though they stretched for a certain distance 

 along a fibril they later separated from it. 



As the growth became older (48 to 72 hours) the cells become more and more 

 densely connected by delicate processes with cells at a distance; and the refractive 

 line of the primitive fibril appeared more and more within the cell and became 

 partly gathered into bundles at one point or another (plate 2, figs. 1 and 2). The 

 cellular cytoplasm became separated into an endoplasm that is, the granular 

 cytoplasm which contains mitochondria, fat, neutral red granules, etc. which 

 immediately surrounds the nucleus, and an exoplasm, or the clear, non-granular 

 cytoplasm of the more remote surfaces of the cell (figs. 10 and 13). 



The delicate fibrils of the cytoplasm continued from one cell to another, usually 

 through the exoplasm of the cell processes (plate 1, fig. 9 and plate 2, fig. 2) and 

 appeared in the living cell as clear, slightly more refractive lines of exoplasm, extend- 

 ing from one cell to another, and frequently across or through the exoplasm of one 

 or more cells. 



As the fibrils developed from day to day the bundle became more definite 

 and more independent of the cytoplasm of the cells, until finally it extended as a 

 slender, clear fiber across several cells (plate 2, fig. 3), and except in cases where the 



