E.E. Just's "independent irritability" revisited: the activated egg as excitable soft matter

Ernest Everett Just's experimental work on post-fertilization events in invertebrate eggs led him to posit a dynamic and directive role for the zygotic "ectoplasm" (cortical cytoplasm), in subsequent development. His perspective was neglected during the years that followed his early death not only because of his well-documented marginalization as an African-American in U.S. science, but because his ideas were at odds with the growing gene-centrism of developmental biology in the latter half of the 20th century. This essay reviews experimental work that shows that the egg cortex in many animal groups is a chemically and mechanically active medium that sustains both spatiotemporal calcium ion transients and periodic deformations in the time leading up to cleavage. These wave phenomena are seen to play regulatory roles in germ plasma localization and gene expression, and influence the reliability and success of developmental outcomes. Just resisted vitalistic explanations for the active processes he observed and inferred regarding the egg cortical cytoplasm, but recognized that the physics and chemistry of his time were inadequate to account for these phenomena and anticipated that expansions of these fields would be necessary to explain them. Here again he proved prescient. Late 20th century developments in the physics of "excitable media" and "soft matter" have provided the bases for models, some of which are described here, of chemical and mechanochemical wave propagation in the activated egg cortex. Lastly, the implications of these post-fertilization phenomena for animal evolution, a problem also addressed by Just, are discussed.