Cephalization and physiological dominance of the anterior region

Sherrington (1906) described reflex arcs from the cephalic receptors. Their fibres end in the gray matter near their own segment, and a second strand carries the path backward to the spinal segments. He regarded the spinal cord, from this point of view, as a mere appendage of the brain¹. Child (1915) reported on flatworms and related forms. In some, head formation stops at a level well away from the cephalic ganglia. In others, no head forms when the ganglia are removed, yet a part of the ganglia left intact, sometimes half or more and sometimes only a small part, allows the head to develop again².

Child (1921) concluded that, at least in lower animals and in early stages, the cephalic part of the central system develops independently of regions behind it. He held that its dominance and its independence are two aspects of one physiological situation³. He explained both by its origin at the high end of the primary gradient, where activity is most intense. He treated the phylogenetic migration of segmental ganglia toward anterior regions as similar in kind, an expression of where activity runs highest⁴.

Lillie (1923) gave a different mechanism. He judged that bioelectric circuits between rapidly growing regions and their neighbours were in all probability an important, if not the chief, factor in physiological dominance⁵. He cited plants, where removing terminal buds starts growth in dormant buds beside them. He added that in certain cases this effect had been shown to be independent of transport of growth-inhibiting substances⁵. Child placed the cause in rate of activity, and Lillie placed it in electrical connection between regions.

Lobo, Beane and Levin (2012) describe the planarian central nervous system as a bi-lobed cephalic ganglia, the brain, joined to two ventral nerve cords⁶. Keijzer (2015) set cephalization in an evolutionary sequence. Nerve nets with long-distance connections allow neurons to be repositioned, and clustering and cephalization then become possible⁷. Fields and Levin (2020) discuss radially symmetric, hypercephalized planarian outcomes. Iglesias et al. observed them for up to 4 weeks after β-catenin RNAi on amputation fragments⁸.