Burrows: Study of Body Cells. 485 



the medium and behave exactly hke the simple undifferentiated 

 mesenchyme cells. In 1912 I ^^ showed, however, that in a few 

 cases these cells which migrate into the medium may differentiate 

 again and develop rhythm. When this takes place it is interesting 

 to note that the cells by chance alone had come into a very peculiar 

 relation with the medium, and again it is interesting to note that this 

 rhythm never develops while the cells are migrating, but always 

 after such migration has ceased and the coagulation process is com- 

 plete. 



The differentiation of the heart-muscle cells in the outer medium 

 takes place very infrequently. In the large majority of the culture 

 of the older embryos and adults the clots cling tightly to the frag- 

 ment. The cells migrate in contact with the surface of the fibrin 

 fibrils. When differentiation takes place the process is different. 

 The clot loosens and contracts away in mass. If the ends of a few 

 cells remain attached to this clot they may become stretched 

 through the serum cavity between the surface of the medium and 

 the end of the fibrin fibrils, or between the fragments and the end of 

 these fibrils. After the coagulation is complete, these cells, and 

 these cells alone, develop rhythm. If they be removed from those 

 contacts and be placed in the outer medium in contact with the 

 surface of the fibrin they stretch out again and behave like simple 

 mesenchyme or connective tissue cells.^ 



By these observations it became possible, therefore, to clear up 

 the difficulty of the earlier observations of the connective-tissue 

 cells. Dynamic states in the organism, such as the heartbeat, are 

 not a property of the cell, but that of a peculiar organization of the 

 environment. These cells may produce the energy for the work of 

 the body, but there is no evidence that the transformation of this 

 energy into work is the product of a cell organization in the case of 

 the heartbeat any more than it had been found to be in the case of 

 cellular migration. 



What is true, for the connective-tissue cells I find also to be true 

 for the epithelial cells of the skin and many of the glands. The 

 gland cells in the cultures lose the form peculiar to them in the or- 

 ganism. They stretch out, like the skin epithelium, to form broad, 

 thin sheets of cells. These cells cannot metabolize except in the 

 presence of fibrinogen or a similar adsorbing substance. They differ 

 from the connective tissue in that they later destroy this substance 

 through certain added proteolytic properties. They thus depend 

 wholly on the fibrinogen for their activity in the culture, but they 

 remain together and form no extracellular fibrils, in that they later 



