GEOMEmiCAL RELATIONS OF CLEAVAGE-FORMS 36; 



and Lamarle have shown, assume a pol^^hedral form and tend toward 

 such an arrangement that the area of surfacccontact /v '-. // 

 a nun^uucn^. Spheres in a mass thus tend o aTsume The £^ Z 

 mterlockm. polyhedrons so arranged that three pCsLterc in 

 a hne, whde four Imes and six planes meet at a point If a ran .cd 

 in a smgle layer on an extended surface, they a'sume the "."'0' 



A 



C 



D 



Fig. 170. — Cleavage ol Polygordius, from life. 



fhpl ^°7-^^" ^*^g^- f'«'" above. B. Corresponding view of eight-cell stage. C Side v.ew of 

 the same (contrast Fig. 169, C) . D. Si.xteen-cell stage from the s^de. 



hexagonal prisms, three planes meeting along a line as before. I^oth 

 these forms are commonly shown in the arrangement of the cells of 

 plant and animal tissues; and Berthold {'^^6) and Errcra {'^6, '%j) 

 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 



