GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS 



269 



leads them to take up the position of least resistance or greatest 

 economy of space. In this regard the behaviour of tissue-cells in 

 general has been shown to conform on the whole to that of elastic 

 spheres, su6h as soap-bubbles when massed together and free to 

 move. Such bodies, as Plateau and Lamarle have shown, assume a 

 polyhedral form and tend towards such an arrangement that the area 



Fig. 121. Cleavage of Polygordius, from life. 



A. Four-cell stage, from above. B. Corresponding view of 8-cell stage. C. Side view of the 

 same (contrast Fig. 120, C). D. Sixteen-cell stage from the side. 



of surface-contact between them is a minimum. Spheres in a mass 

 thus tend to assume the form of interlocking polyhedrons so arranged 

 that three planes intersect in a line, while four lines and six planes 

 meet at a point. If arranged in a single layer on an extended sur- 

 face they assume the form of hexagonal prisms, three planes meeting 

 along a line as before. Both these forms are commonly shown in the 

 arrangement of the cells of plant and animal tissues ; and Berthold 



