Experimental and Theoretical Neurobiology of the Wiring-in of Nervous Systems

Abstract

Facts and artifacts (cf. Hunt and Jacobson, 1974) of experiments dealing with nerve connections were reviewed extensively (Gaze, 1970; Jacobson, 1970b; Hunt and Jacobson, 1974; Young, 1973; Sidman, 1974). If selective nerve connections depend on cell—cell recognitions that involve type II protein ligands (see Section 1.9.1) which link the surfaces of matching cells (Moscona, 1968; Garber and Moscona, 1972), then it is important to distinguish between relatively non-specific ligands, which could act in addition to highly selective ones. Above all one wants to know the genetic control of the postulated type II proteins. Instead of addressing themselves primarily to molecular-biological questions of development, some theorists have preferred a more ‘global’ approach to cellular ordering based on ‘developmental gradients’ and allied concepts (Dalcq, 1938; Child, 1941; Sperry, 1955, 1965; Wolpert, 1969, 1971; Wolpert, Hicklin and Hornbruch, 1971, and various papers in Symp. Soc. Exptl. Biol. 25; also Summerbell, Lewis and Wolpert, 1973). Some theorists, as noted, postulated prepatterns. By avoiding detailed molecular-biological explanations, global developmental theories have disregarded possible mechanisms for the genetic control of development. Such mechanisms are, at present, no more speculative than gradient theories and allied ideas. Several genetic control theories of development, based on molecular biology, have been proposed (Davidson and Britten, 1971; Britten and Davidson, 1969; Strehler et al., 1971; Wassermann, 1972, 1973), although only the ideology of Wassermann (1972, 1973) involves type II differentiations. Type II differentiations were suggested by Weiss (1947; see also Weiss, 1968), but not in relation to genetics.