The origin of slaty cleavage is in some measure obscure.
The connexion of cleavage with pressure, however, is unmistakable.
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 cleavage of the ovum follows two types, both seen in Tubularia (Brauer ).
It will be observed that the cleavage planes do not pass through the white bands.