cleavage
1. In minerals, cleavage is evident when crystals split along planes of weakness inherent in the structure of their atomic lattices. Cleavage is described by an adjective, e.g. good, poor, etc., and by referring to its crystallographic direction, plane, and degree of perfection, the resulting digits being contained in braces ({}) to distinguish them from descriptions of crystals. See MILLER INDICES.

2. The formation of a set of fractures along closely spaced, parallel surfaces in a rock (the term is usually applied to low-grade metamorphic rocks) by the alignment of various mineralogical and structural elements during metamorphism and deformation, e.g. in slates, where cleavage is due to a parallel arrangement of minerals. The fabric generally gives rise to a preferred direction of fracturing, broadly analogous to mineral cleavage. Rock cleavages may be divided into two groups: (a) continuous cleavages, e.g. ‘slaty cleavage’ (synonymous with schistosity and foliation in high-grade metamorphic rocks, see METAMORPHIC GRADE) which, with further deformation, may be superimposed and cross-cut by a secondary crenulation cleavage; (b) spaced cleavages, either crenulation or disjunctive, e.g. fracture cleavage. Crenulation cleavages form by the microfolding of a pre-existing anisotropic fabric. Disjunctive cleavages require no such primary fabric. Compare FOLIATION.

cleav·age / ˈklēvij/ • n. a sharp division; a split. ∎  the hollow between a woman's breasts when supported, esp. as exposed by a low-cut garment. ∎  Biol. cell division, esp. of a fertilized egg cell. ∎  the splitting of rocks or crystals in a preferred plane or direction.

cleavage (furrowing, segmentation) The process by which a dividing egg cell, following fertilization, gives rise to all the cells of the organism. In animals, this division forms a cellular mass called a blastula. If cleavage follows a definite pattern it is said to be determinate (and hence permits the trading of cell lineages). In some species, however, the pattern is lost after the first few cell divisions.

cleavage In embryology, progressive series of cell divisions that transform a fertilized egg into the earliest embryonic stage (blastula). The egg is divided into blastomeres (smaller cells), each containing a diploid number of chromosomes.

cleavage (in embryology) The series of cell divisions by which a single fertilized egg cell is transformed into a multicellular body, the blastula. Characteristically no growth occurs during cleavage, the shape of the embryo is unchanged except for the formation of a central cavity (the blastocoel), and the ratio of nuclear material (DNA) to cytoplasm increases.