Homoeotic Gene ETc Controls the Cell Number of Embryonic Ectodermal Layers of Bombyx mori

The formation of segments of Bombyx larvae involves differentiation of legged and legless segments during embryogenesis. Observations by light microscopy of serial sections of developing embryos of Bombyx showed that the cell number of the ectodermal layers increased more rapidly in segments where legs were being extruded than in those where no appendages were formed. In the embryos of a homoeotic mutant for the E -pseudoallelic locus (about 0.0–VI), E^Tc/E^Tc , in which all the abdominal segments were legless, the cell number of the ectodermal layers did not increase as in normal embryos. These findings suggest that the E^Tc gene controls the cell number of the ectodermal layers in relation to the differentiation of abdominal segments.