174 PAUL WEISS 



phology of inhabitant, as well as immigrating, cells. Potent as these 

 effects are, they could not be a major factor in development if only 

 tensions of extraneous origin were involved. But tensional stresses also 

 arise within the developing organism as a result of shifts, growth, and 

 chemical activity of its parts. 



Figure i. Diagram illustrating the effect of graded stretching (in the direction of 

 the arrows) of a reticular matrix on the shape of mesenchymal cells. 



Let us consider the two main intrinsic sources of morphogenetic ten- 

 sions — expansion and contraction. Their development is illustrated in 

 Figure 2. On the left side (Ao, Bo) are sectors of a tissue possessing a 

 continuous reticulated matrix and containing a circumscribed area 

 (dark circle) in which some localized developmental process goes on, as 

 follows. The lower half (Bo-^Be) represents a case in which the marked 

 area expands relative to its surroundings, either by rapid proliferation 

 or by deposition of large amounts of ground substance or, in the case of 

 hollow organs, by distension. This local expansion creates in the sur- 

 rounding continuum tensions that are oriented circumferentially, de- 

 clining in strength with increasing distance from the border. The fibrous 

 matrix assumes a corresponding pattern as depicted, and this in turn 

 imparts itself to the cells. Cell movements and cell divisions will follow 

 tangential courses. It is evident that the familiar concentric architecture 

 of the mesenchymal sheaths, coats, tunics, and membranes around both 

 solid organs and hollow ducts and vesicles finds a natural explanation on 

 this basis. Essentially the same effect is observed if the surrounding tis- 

 sue contracts against a rigid core, as in the formation of foreign-body 

 capsules. 



A wholly different pattern results if a tissue contains a focal area 

 that contracts. In this case, illustrated in the top row of diagrams 

 (Ao-^Ac), the matrix adhering to the shrinking area is subject to radial 

 tensions, and the surrounding tissue will thus be gathered into a star- 



