cleavage
(ˈkliːvɪdʒ)
[f. cleave v. + -age.]
1. a. The action of cleaving or splitting crystals and certain rocks along their lines of natural fissure; the state of being so cleft.
| 1816 Cleaveland Min. 9 The primitive forms of crystals can be ascertained only by mechanical division. This process, sometimes called cleavage by lapidaries, consists in separating thin layers or slices from the sides, edges, or angles of a crystallized substance in a given direction. 1831 Brewster Optics xvii. §90. 145 We may by a new cleavage replace the imperfect face by a better one. |
b. Min. Arrangement in laminæ which can be split asunder, and along the planes of which the substance naturally splits; fissile structure; the property of splitting along such planes.
| 1830 Sir J. Herschel Stud. Nat. Phil. 291 The texture or cleavage of a mineral. 1869 Roscoe Elem. Chem. 191 Crystalline bodies exhibit..a peculiar power of splitting in certain directions more readily than in others, called cleavage. |
c. Geol. slaty cleavage: the fissile structure in certain rocks, especially in clay slate and similar argillaceous rocks, whereby these split into the thin laminæ or ‘slates’ used in roofing, etc. This structure is quite distinct from, and in origin posterior to, the stratification and jointing, the cleavage-lines crossing these at any and every angle, while parallel to themselves over extensive tracts of country.
| 1839 Murchison Silurian Syst. 574 The observation of Professor Sedgwick on the slaty cleavage of mountains. 1845 Darwin Voy. Nat. vi. 116 A formation of quartz which..had neither cleavage nor stratification. 1860 Tyndall Glac. 2, I learned that cleavage and stratification were..totally distinct from each other. 1882 Geikie Text-bk. Geol. 121 This superinduced fissility or ‘cleavage’ has resulted from an internal rearrangement of the particles in planes perpendicular to the direction in which the rocks have been compressed. |
d. (with pl.) The direction or plane in which a crystal or rock may be split.
| 1817 B. Jameson Char. Min. 135 The cleavages are not parallel with any of the planes of the crystal. 1869 Tyndall Light 73 By following these three cleavages it is easy to obtain from the crystal diamond-shaped laminæ of any required thinness. |
e. Biol. Cell-division, segmentation.
| 1875 E. R. Lankester tr. Haeckel's Hist. Creation I. 190 Commencement of the development of a mammal's egg, the so-called ‘cleavage of the egg’ (propagation of the egg-cell by repeated self-division). 1879 tr. Haeckel's Evol. Man I. 185 The parent-cell, which arose from the fertilized egg-cell, separates, by repeated cleavage, into a large number of simple cells. 1896 E. B. Wilson Cell 143 During the early anaphase of the first cleavage each centrosome divides into two. 1902 Encycl. Brit. XXVIII. 140/1 Segmentation or cleavage of the ovum. 1948 New Biol. V. 112 The usual process of cleavage is as follows. The egg nucleus divides by mitosis; [etc.]. |
2. a. gen. The action or faculty of cleaving or splitting asunder; the state of being cleft; division. lit. and fig.
| 1867 Froude Short Stud., Erasm. & Luther (ed. 2) 26 When differences of religious opinion arose, they split society to its foundation. The lines of cleavage penetrated everywhere. 1879 Baring-Gould Germany I. 60 We lament, in England, the cleavage between the classes. 1886 Spurgeon Treas. Dav. Ps. cxliii. Introd., This psalm is divided by the Selah. We prefer to follow the natural cleavage, and therefore have made no other dissection of it. |
b. The cleft between a woman's breasts as revealed by a low-cut décolletage. colloq.
| 1946 Time 5 Aug. 98 Low-cut Restoration costumes..display too much ‘cleavage’ (Johnston Office trade term for the shadowed depression dividing an actress' bosom into two distinct sections). 1947 Landfall I. 45 It [sc. a film] just goes all out to exploit sex and violence as blatantly as it can, with the result that ‘cleavage’ has once again become a problem to haunt the dreams of censors. 1958 Spectator 6 June 729/1 Kids have to learn not to copy the stars. I tell them, cleavage won't get you to the top. Sex is something different. It's not obvious. 1958 TV Times 10 Oct. 21/2, I was foolish enough to wear the sort of dress which showed vistas of cleavage. |
3. attrib. cleavage-cavity = blastocœle; cleavage-cell, -globule = blastomere; cleavage-mass, (a) = blastomere; (b) a mass of rock formed by cleavage; cleavage-nucleus, the nucleus of the fertilized egg resulting from the union of male and female pronuclei; cleavage-plane (see quot. 1878); cleavage-spindle, the karyokinetic spindle of the dividing blastomere.
| 1831 Brewster Optics xxv. 214 Analcime has certainly no cleavage planes. 1862 Dana Man. Geol. 55 A broad, even, lustrous cleavage-surface. 1871 Huxley Anat. Vert. 10 The cleavage-masses eventually become very small, and are called embryo-cells. 1875 Dawson Dawn of Life v. 117 The loganite..shows traces of cleavage-lines. 1878 Gurney Crystallog. 8 Most crystals can be separated into indefinitely thin slices, which are bounded by flat surfaces called cleavage-planes. 1879 tr. Haeckel's Evol. Man I. 186 Cleavage-cells or Cleavage-globules (segmentella). Ibid. I. 189 The inner cavity of the ball, which is filled with clear liquid or jelly, is called the cleavage-cavity (cavum segmentarium), or the germ-cavity (blastocœloma). 1882 Geikie Text-bk. Geol. ii. ii. §6. 121 ‘Clay-slate’ has generally been applied solely to argillaceous rocks possessing this cleavage-structure. 1887 Encycl. Brit. XXII. 425/2 In this stage thin sections show that the cleavage cavity is obliterated. 1896 E. B. Wilson Cell 156 The first cleavage-nucleus. 1912 Brit. Museum Return 196 Enargite, large cleavage masses from Red Mtn., Colorado. Ibid. 157 The centrosomes of the cleavage-spindle. |