52 DEVELOPMENT OF CONNECTIVE-TISSUE FIBERS. 



hours and continue to develop until the growth is about 6 days old or over. 

 Cultures of the subcutaneous tissue from an 11 or 12 day chick embryo also prove 

 very satisfactory for study, for not only is the new growth available for study, 

 but the explanted piece itself is so thin that the cells and fibers can be observed 

 even with the oil -immersion lens. 



The fibers in the explanted piece were not observed to grow either in length 

 or bulk, and after a period of two weeks they remained much the same as when 

 explanted. In no case was a fiber observed to pass out from an explanted piece 

 over the new growth; such a fiber always remained curled up within the explanted 

 piece. 



New fibrils begin to develop in the new growth from an explanted piece of 

 tissue from an 8 to 12 day chick embryo shortly after the new growth is 24 hours 

 old, and definite bundles of fibrils may be developed when the growth is 5 or 6 

 days old. These fibrils develop more quickly in growths from the older embryos 

 of 10 to 12 days than in those from the younger embryos of 8 to 10 days. 



The new growth from the subcutaneous tissue is extremely sensitive and 

 reacts to all sorts of changes in its environment, by contraction. Frequently 

 while a membrane of connective tissue was under observation it would begin to 

 contract from the outer edge of the growth and draw in towards the explanted 

 piece. This contraction might stop at any period or it might continue until the 

 entire new growth had contracted close to the explanted piece. The explanted 

 piece was never observed to contract. The relaxation after such a contraction 

 was exceedingly slow, and frequently a contraction that had taken no longer than 

 2 to 5 minutes required for relaxation from 1 to 6 hours. In fact, the process 

 did not resemble relaxation, but rather a growing-out again of the new growth. 

 Often, coincident with the contraction, there occurred a rolling-back of the edge 

 of the growth, and in this case when the cells migrated out again many of them 

 became changed in their relative positions. Thus it is evident that a decided 

 strain is present during the development of the fibrils, though there is no fibrin 

 and (so far as can be seen) no substance which coagulates surrounding the cells. 

 Whether this strain or tension (often exhibited by the contraction above described) 

 may in any way influence the separation of the fibrils from the cytoplasm of the 

 cell, it is impossible to state. It was not certain that a preparation which con- 

 tained well-developed fibrils had not contracted during the development of the 

 fibrils. However, it can be definitely stated that no substance formed into fibrils 

 during contraction, as might have been expected from the experiments of Loeb 

 and of Isaacs. The new growth, when relaxed after such a contraction, never 

 contained any suddenly formed fibers or fibrils, and such fibrils as were present 

 were in very much the same state of development as that in which they were 

 before the contraction took place. Also, many membranes in which no fibrils 

 ever developed possessed the power to contract, and did contract more than once. 



From these general observations it is evident that the fibers which form in 

 the tissue cultures must arise from the cells; and since the cells are spread out in 

 a thin layer the process of development of the fibers can be observed in the living 

 cell undisturbed by any manipulation. 



