GROWTH OF TISSUES OF THE CHICK EMBRYO 79 



contraction of the surrounding fibrin. When this occurs over a 

 growing tissue, the cell layer is broken. The cells become spindle- 

 shaped and form concentric rings about the border of the opening. 

 These cells frequently increase in number. They constantly re- 

 tain their shape, however, except at the time of active karyoki- 

 nesis when they assume a rounded form, as may be readily seen 

 in stained preparations. 



The question of an active force influencing the growth of these 

 cells is suggested by the long spindle form of the cells growing 

 from the cut ends of a beating heart. In quiescent hearts these 

 cells are polygonal or oval in form. In the specimens where the 

 hearts are transplanted to the same drop of plasma in which a 

 growing mesenchyme tissue is present, the cells of the tissue lying 

 along the line of the transmitted force of the heart beat are seen 

 as long parallel rows of spindle cells. 



To aid in the study of the effect of outward force on the cell and 

 the fibrin, a heart was injured on its convex surface. The ends 

 were brought together and held until clotting had taken place. 

 In a few hours the heart was entireh^ free from the clot except 

 at its cut ends and the opposite injured point on the convex surface. 

 The force of each beat was transmitted directly to the fibrin in 

 a straight line across the drop. After five days, along this line, 

 was a thickened ridge of fibrin composed of long parallel coarse 

 fibrillae. The cellular debris and the small pieces of the tissue 

 had been drawn into it from neighboring parts and arranged in 

 rows parallel to these fibrillae. Preexisting and growing cells in 

 this zone alike assumed a spindle form. The heart's force was 

 undoubtedly associated with this change in type of fibrin and of 

 cell. 



Here, as in the above observations, one can readily associate 

 these cells with this definite fibrin architecture. Whether their 

 form or movement is in any way due to contractions of the fibrin 

 we cannot say, but from the present observations one might as- 

 sume that through their own activity these cells adapt themselves 

 to the variety of the support at hand. 



