The length scale of separation is conserved among different species. (A) Schematic illustration of nuclear separation (top) and proposed activities (bottom). After chromosome segregation in anaphase, the central spindle disassembles (blue), and microtubule (MT) asters (green) linked to F-actin (yellow) transport daughter nuclei further apart. This transport continues until sister centrosomes have reached opposite sides of the nucleus (green line in middle graph). This diametric centrosome positioning cancels migration of the sister asters, and nuclear movement ceases. The internuclear distance defines a nuclear domain (gray) of characteristic size. (B) Scheme of a hypothetical, small-sized syncytium with four dividing nuclei, illustrating the principle of nuclear distribution. The number of divisions and the size of the nuclear domain (dashed circles) determine when nuclei reach the cortex by filling cytoplasmic space. (C) Logarithm of embryo volume as a function of the observed number of preblastoderm divisions for different insect species undergoing a syncytial phase of development in embryos of different size (see Table 1). Regression analysis predicts a domain size in the range of 28.1 and 31.3 µm with 95% confidence. For three species, an embryo is drawn to scale and positioned on the x axis.