The origin of slaty cleavage is in some measure obscure.
They lose their facile cleavage and become hard, dark-coloured, slightly lustrous rocks, which have a splintery character or break into small cuboidal fragments.
The connexion of cleavage with pressure, however, is unmistakable.
The cleavage of the ovum follows two types, both seen in Tubularia (Brauer ).
It consists in an unequal number of chromosomes passing over to each of the daughter nuclei, so that one may become hypochromatic, the other hyperchromatic. When this happens the resulting cleavage of the cytoplasm and nucleus is also unequal.