176 PAUL WEISS 



arises around actively proliferating centers through the following chain 

 of events. Cells in proliferation, and apparently only then, liberate agents 

 with strong dehydrating potency. The result is a progressive condensa- 

 tion of the surrounding fibrous phase of the medium. Such local con- 

 densation in a rigid tissue frame generates tensions, which in turn 

 orient the condensing system toward the center of shrinkage. The fa- 

 miliar ray-shaped growth of ordinary tissue cultures in blood plasma is 

 due to this efifect. Analogous configurations can be recognized in many 

 embryonic formations. By its radial orientation such a contraction pat- 

 tern can evidently guide peripheral cells toward the center. This is one 

 of the major mechanisms by which cells are drawn toward distant des- 

 tinations, as if "attracted." Its morphogenetic importance becomes par- 

 ticularly impressive when several such centers act on a common matrix, 

 where they create interconnections of great regularity among the other- 

 wise independent centers. As is readily understood from Figure 3, two 



Figure 3. Diagram illustrating the orientation of a reticular matrix along the con- 

 necting line between two centers of contraction. 



contractile centers produce the greatest amount of shrinkage, hence ten- 

 sion, and thereby orientation of the matrix along their connecting line; 

 this automatically establishes a bridge for cell traffic between the centers. 

 Three centers determine a triangle (Plate IV), and so forth. In prin- 

 ciple, these experiments have solved the problem of orientation in cell 

 movement and growth. 



In acknowledging mechanical stress as a major morphogenetic factor, 

 two reservations must be borne in mind. Firstly, tension is not the only 

 agent capable of orienting tissue structure. Any other physical force that 

 is capable of afifecting the orientation and aggregation of polar molecules 

 (electrostatic fields, electrophoresis, streaming, etc.) may have com- 



