By studying the cleavage in the eggs of different animal groups, it has been concluded that in all animal eggs, the cleavage seems to be governed by certain basic principles, rules of laws. Certain fundamental rules or laws of cleavage are following:
1. Sach’s law- In 1877, Sach proposed following two laws:
a) Cells tend to divide into equal daughter cells.
b) Each new division plane tends to intersect the preceding plane at right angles. (Acts to maintain) the spheroidal shape of blastomeres).
2. Hertwig’s Laws- In 1881, O. Hertiwig added following laws of cleavage in Sach’s laws:
a) The nucleus and achromatic figure (or mitotic) spindle occupy the “centre of protoplasmic density” of the egg or blastomeres in which it lies. (Hence, in the microlecithal and isolecithal eggs, the spindle is located centrally; in a telolecithal ovum it is nearer the animal pole).
Corollary- Blastomeres divide into two equal parts unless the yolk is unevenly stored in them.
b) The axis of a mitotic spindle occupies the longest axis of the protoplasmic mass in which it lies and division therefore tends to cut this axis transversely (evident in ovoid blastomeres).
Corollary- The ensuing plane of division cuts across the long axis, and the daughter cells revert to a more spheroidal shape.
3. Balfour’s law- Balfour’s law which was formulated by Balfour in 1885, states that the speed or rate of cleavage in any region of egg is inversely proportional to the amount of yolk or deutoplasm it contains. (In telolecithal eggs, blastomeres at the animal pole divide faster than those nearer the vegetal pole)