Burrows: Study of Body Cells. 493 



energy necessary for it from the energy-producing reaction of the 

 cell. The extracellular fibers of the connective tissue are not secre- 

 tions of the cells any more than bone or cartilage are of this origin.^ 

 These fibers are the combination of proteins formed elsewhere in the 

 body and the ''L" substance of these cells. 



There is no reason to believe, therefore, that protein synthesis is 

 a part of the metabolism of the cell. It is something different. It 

 is a form of work produced. Upon it growth depends. Growth has 

 never been shown to be of a simple chemical or physical nature. 

 It is the result of the careful utilization of energy. It takes place 

 against the forces of nature. Protein synthesis, like muscular con- 

 traction, is only a form of work peculiar to body organizations and 

 not cellular organizations. 



In 1917,^*^ and again in recent experiments,^ I have shown that 

 the development of ryhthmical contraction is not associated with 

 any fundamental change in the cell. This is a property peculiar to 

 any of the mesenchyme cells of early embryonic life. It occurs in 

 the fragments of this tissue and in the cells which migrate from 

 them. Its development is the result of a chance relationship of the 

 cells to medium. This relation is wholly dependent upon the 

 physical peculiarities of the coagulation of the plasma clot. The 

 isolated cells which develop rhythmical contraction are those cells 

 which become stretched through a serum cavity between the sur- 

 face of the medium or a cellular tissue fragment and the end of 

 fully formed fibrin fibrils. No contractions develop until these 

 fibrin fibrils are fully formed. What is true for the isolated cell 

 is also true for the fragments. Fragments of the heart of young 

 embryos contract at once when removed to warm medium of 

 the culture. Those from older embryos fail to show this change. 

 Rhythm develops in these latter fragments only after the border 

 cells have moved out or the process of coagulation is completed. 

 The end of the cell in contact with the fibrin is in metabolic equi- 

 librium. These cells imbedded in fully formed clot cease all ac- 

 tivity. The cells floating in serum also show no change. The only 

 active part of these cells differentiated for contraction is the free 

 end in contact with the cellular fragment or the surface of the 

 medium. This end suffers a decrease in surface tension. Such a 

 decrease in surface tension is associated with electrical changes at 

 this end of the cell and a stretching of the cell. If such changes 

 continue, one of three conditions must result: the cell will be torn 

 loose or in two, or there will be an explosive breakdown of this 



