cleavage

/klee"vij/, n.

1. the act of cleaving or splitting.

2. the state of being cleft.

3. the area between a woman's breasts, esp. when revealed by a low-cut neckline.

4. a critical division in opinion, beliefs, interests, etc., as leading to opposition between two groups: a growing cleavage between the Conservative and Liberal wings of the party.

5. the tendency of crystals, certain minerals, rocks, etc., to break in preferred directions so as to yield more or less smooth surfaces (cleavage planes).

6. Embryol. the total or partial division of the egg into smaller cells or blastomeres.

7. Also called scission. Chem. the breaking down of a molecule or compound into simpler structures.

[1810-20; CLEAVE² + -AGE]

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Tendency of a crystalline substance to split into fragments bounded by plane surfaces.

Cleavage surfaces are seldom as flat as crystal faces, but the angles between them are highly characteristic and valuable in identifying a crystalline material. Cleavage occurs on planes where the atomic bonding forces are weakest; for example, galena cleaves parallel to all faces of a cube. Cleavage is described by its direction (as cubic, prismatic, basal) and by the ease with which it is produced. A perfect cleavage produces smooth, lustrous surfaces. Other degrees include distinct, imperfect, and difficult. See also fracture.

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▪ embryo

      in embryology, the first few cellular divisions of a zygote (fertilized egg). Initially, the zygote splits along a longitudinal plane. The second division is also longitudinal, but at 90 degrees to the plane of the first. The third division is perpendicular to the first two and is equatorial in position. These early divisions produce separate cells called blastomeres. The first few cleavages occur simultaneously in all of the blastomeres (cells), but, as the number of cells increases, simultaneity is lost, and the blastomeres divide independently. Little growth occurs between divisions. Even after several divisions, the group of blastomeres is about the same size as the original zygote. Only new chromatin (nuclear material) is synthesized between divisions, and this takes place at the expense of the cytoplasm (the substance of the cell outside the nucleus).

      The pattern of cleavage varies among animal groups but is quite standard for all individuals in a given species. Those eggs such as birds' eggs that contain much yolk often do not divide completely through the yolk-rich region and are called meroblastic. The blastomeres in the yolk-free region cleave completely and result in the embryo proper, while peripheral blastomeres become the yolk sac. Eggs with little yolk divide completely and are termed holoblastic.

▪ mineralogy

      tendency of a crystalline (crystal) substance to split into fragments bounded by plane surfaces. Although cleavage surfaces are seldom as flat as crystal faces, the angles between them are highly characteristic and valuable in identifying a crystalline material.

      Cleavage occurs on planes where the bonding forces are weakest. A crystal may be cleaved with equal ease in any direction that is parallel to crystallographically identical faces; for example, galena cleaves parallel to all faces of a cube. Cleavage is described by its direction (as cubic, prismatic, basal) and by the ease with which it is produced. A perfect cleavage produces smooth, lustrous surfaces with great ease. Other degrees include distinct, imperfect, and difficult. See also fracture.

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