Cytoplasmic localization and prelocalized egg regions

Wilson (1896) reviewed a group of views that, with the exception of Roux's, leaned toward one conclusion. The egg's cytoplasm is mapped out from the start into regions that correspond with parts of the future body.¹ On this account cleavage does not create the regions. It only marks off their boundaries.² Wilson added that the arrangement in the egg need not match the adult arrangement. It involves the adult parts as a consequence, as one kaleidoscope picture leads to the next. In tunicate and cephalopod eggs, Wilson said, adult bilateral symmetry is foreshadowed even before cleavage.¹ Verworn (1899) quoted a passage that applies His's principle of organ-forming germ regions and calls gastrulation a mosaic work. Verworn left the reader to judge how far Roux was a preformationist.³

Loeb (1916) argued that the egg, or rather its cytoplasm, is the future embryo. The Mendelian factors in the chromosomes can impress only individual characteristics on it.⁴ Loeb noted that an egg can be made to develop without a spermatozoon, but a spermatozoon cannot be made to develop without egg cytoplasm.⁴ As evidence, Loeb cited sea urchin development up to the gastrula, which is independent of the nucleus. The nucleus shows its influence only once skeleton or mesenchyme forms.⁵ This rested on Boveri's experiments with an enucleated egg fragment fertilized by a spermatozoon of a foreign species.⁶ Loeb drew a conditional conclusion. If this holds generally, genus characters and possibly species characters are set by egg cytoplasm.⁵

Child (1924) disputed the use of mosaic development as proof of predetermination. Parts that develop and differentiate independently up to a point had often been taken to show it.⁷ Child held that all the evidence points otherwise. Mosaic development and self-differentiation are, with certain exceptions, secondary conditions.⁷ Child pointed to developmental processes that stand in direct relation to factors inside or outside the organism. When these relations are altered, for example by isolating parts, the parts respond with altered development.⁸ Wilson and Loeb placed the map in the egg. Child placed the explanation in the relations among parts and their surroundings.

Levin (2023) gives a table that pairs features of cognition with features of bioelectric tissue. One entry lists representation of patterning goal states by bioelectric properties of tissue.⁹ In this framing the pattern is held in the physiological state of tissue. The passage is a list and does not mention egg regions or cytoplasmic localization. Whether such tissue states relate to the older egg maps is not settled in the passages held here.