cleavages

Cleavages, or cleavage, is a term used in several disciplines to describe processes of division or separation, though its exact meaning varies by field.

In biology, cleavage refers to the rapid mitotic divisions that follow fertilization, producing a multicellular embryo without overall growth in size. The early stages include the two-cell, four-cell, morula, and blastula. Cleavage patterns vary among species and are described as holoblastic (complete division) or meroblastic (incomplete division, often due to yolk). In animal cells, cytokinesis is marked by the formation of a cleavage furrow, a contractile actin ring that constricts the cell membrane to divide the cytoplasm.

In geology and mineralogy, cleavage describes a mineral or rock's tendency to break along flat, planar surfaces that reflect internal grain orientation. Cleavage planes can be perfect, good, or indistinct, and their presence aids in rock identification and influences mechanical properties such as strength and weathering behavior. Some rocks exhibit schistose or slaty cleavage, reflecting progressively aligned minerals due to metamorphism.

In sociology and political science, cleavages are enduring social divisions that shape political behavior, party systems, and policy preferences. Common cleavages include class, religion, ethnicity, and urban–rural or center–periphery divides. The study of cleavages explains patterns of voter alignment and how institutions mediate the impact of social divisions on politics; a notable framework is Lipset and Rokkan’s cleavage theory.

In plant biology, the term is less commonly used beyond embryology, but plant cytokinesis involves vesicle fusion forming a cell plate that ultimately divides the cytoplasm between daughter cells, completing cell division.