GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS 



367 



and Lamarle have shown, assume a polyhedral form and tend toward 

 such an arrangement that the area 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 surface, they assume the form of 



C D 



Fig. 170. Cleavage of Polygordius, from life. 



A. Four-cell stage, from above. B. Corresponding view of eight-cell stage. C. Side view ol 

 the same (contrast Fig. 169, C) . D. Sixteen-cell stage from the side. 



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 ('86) and Errera ('86, '87), 

 carefully analyzing the phenomena, have endeavoured to show that 

 not only the form and relative position of cells, but also the direction 

 of cell-division, is, partially at least, thus determined. 



It is through displacements of the cells of this type that many of 



