cleavage (redirected from cleaving)

cleavage, tendency of many minerals to split along definite smooth planar surfaces determined by their crystal structure. The directions of these surfaces are related to weaknesses in the atomic structure of the mineral and are always parallel to a possible crystal face. The property of cleavage is useful in identifying a mineral species. The tendency for certain varieties of metamorphic and sedimentary rock to split along more or less smooth surfaces is sometimes referred to as rock cleavage. Flagstone, slate, and schist are noted for this property, which arises from the parallel alignment of fine, platy mineral grains, themselves displaying cleavage.

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cleavage

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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cleavage

1. (of crystals) the act of splitting or the tendency to split along definite planes so as to yield smooth surfaces

2. Embryol (in animals) the repeated division of a fertilized ovum into a solid ball of cells (a morula), which later becomes hollow (a blastula)

3. the breaking of a chemical bond in a molecule to give smaller molecules or radicals

4. Geology the natural splitting of certain rocks, or minerals such as slates, or micas along the planes of weakness

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Cleavage (embryology)

The subdivision of eggs into cells called blastomeres. It occurs in eggs activated by fertilization or parthenogenetic agents. Cleavages follow one another so rapidly that there is little opportunity for daughter cells to grow before they divide again. Consequently the size of blastomeres diminishes progressively, although many times unequally, during cleavage. By contrast, the nucleus of each daughter cell enlarges following each cleavage with the result that the ratio of the volume of the nucleus to the volume of cytoplasm (the nucleoplasmic ratio) progressively increases. The cleavage period is said by some authorities to terminate when the nucleoplasmic ratios of various blastomeres attain values characteristic of adult tissues. Cells continue to divide thereafter, but each daughter cell then undergoes a period of growth prior to its division with the result that the nucleoplasmic ratio tends to remain approximately constant for each cell type following termination of cleavage. According to others, cleavage terminates with formation of the definitive blastula. Cleavage appears to be an essential step in development. Although some differentiation occurs in eggs of certain animals when cleavage is blocked experimentally, it is limited and infrequent. See Blastulation

Cleavage does more than merely subdivide the substance of the egg quantitatively into smaller units, the blastomeres, which are then of such a size that they can readily undergo the subsequent events of blastulation, gastrulation, and interaction that are involved in formation of tissues and organs. Sooner or later, cleavage segregates different cytoplasmic areas into different blastomeres, thus subdividing the substance of the egg qualitatively. These qualitative cytoplasmic differences among blastomeres are then sufficient to account for the initial establishment of different lines of differentiation in the progeny of different blastomeres, even though the genetic content of all blastomeres is identical. See Cell lineage