1650s, "second stomach of a ruminant" (so called from the folds of the membrane), from Latin reticulum "a little net" (see rete). The word was later given various uses in biology, cytology, histology, etc., and made a southern constellation by La Caille (1763).
"having no brain" (biology), 1821, with -ic + Latinized form of Greek anenkephalos, from an- "not, without" (see an- (1)) + enkephalos "brain," "the brain," literally "within the head," from en "in" (see en- (2)) + kephalē "head;" see cephalo-. Related: Anencephalous (1834); anencephalia; anencephaly.
1610s, "mold or matrix in which anything is cast or formed to a particular shape" (a sense now obsolete); see plasma. In biology, the meaning "living matter of a cell, protoplasm" is attested by 1864.
"the number two, two units treated as one," 1670s, from Latin dyad-, stem of dyas, from Greek dyas "the number two, a group of two," from duo "two" (from PIE root *dwo- "two"). Specific sense in chemistry ("a bivalent element") is by 1865; also used in biology, poetics, mathematics. Related: Dyadic.
as a noun, in biology, "genetic variant of an animal," 1955; as a verb, in cinematic special effects, c. 1987, short for metamorphosis. Related: Morphed; morphing. Earlier it was a slang shortening of morphine (1912).
1824 in biology, "science of the outer form and inner structure of animals and plants," from German Morphologie (1817); see morpho- "shape" + -logy "study of." By 1869 in philology, "science of structure or forms in language." General sense of "shape, form, external structure or arrangement" is by 1890. Related: Morphological; morphologist. Related: Morphologist.