Further Development 2.2: Squeezing the Conditions of Specification

Driesch confirmed conditional development in sea urchin embryos with an intricate recombination experiment. If in fact some nuclear determinant dictates a cell’s fate (as proposed by Weismann and Roux), then changing how nuclei are partitioned during cleavages should result in deformed development. In sea urchin eggs, the first two cleavage planes are normally meridional, passing through both the animal and vegetal poles, whereas the third division is equatorial, dividing the embryo into four upper and four lower cells (Figure 1A). Driesch (1893) changed the direction of the third cleavage by gently compressing early embryos between two glass plates, thus causing the third division to be meridional like the preceding two. After he released the pressure, the fourth division was equatorial. This procedure reshuffled the nuclei, placing nuclei that normally would have been in the region destined to form endoderm into the presumptive ectoderm region. In other words, some nuclei that would normally have produced ventral structures were now found in the dorsal cells (Figure 1B). Driesch obtained normal larvae from these embryos. If segregation of nuclear determinants had occurred, then this recombination experiment should have resulted in a strangely disordered embryo. Thus, Driesch concluded that “the relative position of a blastomere within the whole will probably in a general way determine what shall come from it.”