1610s, "arriving at puberty," a back-formation from pubescence or else from French pubescent (early 16c.) or directly from Latin pubescentem (nominative pubescens), present participle of pubescere "to reach puberty." By 1760 in biology, "covered with fine short hairs or down."
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.