“Mosaic work” and “assimilating effects” in embryogenesis: Wilhelm Roux’s conclusions after disabling frog blastomeres

Abstract

Wilhelm Roux, after killing inividual frog blastomeres, conceived the mosaic theory of embryonic differentiation. At least, that is what the textbooks say. However, this essay will show that Roux claimed self-differentiation (or mosaicism) of blastomeres only for normal — in his terminology “ typical” — development, and strictly speaking only for its earliest stages, while seeing ample proof of interactive mechanisms in abnormal (or “atypical”) development after experimental interference. He thus both realized and stated that mechanisms other than mosaic development must exist, and he assumed a role for them in normal development as well; whatever he may have written about mosaic development, he never claimed that this was the only mechanism at work. Indeed, in his first communication on puncturing frog eggs he clearly stated that developmental interaction was required even in self-differentiation: “It should not be necessary to stress again that differentiation, considered as such, is a product of interaction; and that, when distinguishing independent from dependent differentiation, what matters solely is to establish whether the specific cause of a [developmental] change is located within the part undergoing the visible change or outside of it” (Roux 1885, p. 523).