universally true for cells generally is shewn, for instance, by such well-known cases {348} as the unequal segmentation of the frogãs egg. It is true when the dividing cell is homogeneous, and under the influence of symmetrical forces; but it ceases to be true when the field is no longer dynamically symmetrical, for instance, when the parts differ in surface tension or internal pressure. This latter condition, of asymmetry of field, is frequent in segmenting eggs1, and is then equivalent to the principle upon which Balfour laid stress, as leading to ãunequalã or to ãpartialã segmentation of the egg,ãviz. the unequal or asymmetrical distribution of protoplasm and of food-yolk.
The second rule, which also has its exceptions, is true in a large number of cases; and it owes its validity, as we may judge from the illustration of the repeatedly bisected cube, solely to the guiding principle of minimal areas. It is in short subordinate to, and