Morphogenesis in the early embryo of the lepidopteran Bombyx mori

The regional patterns of nuclear and cell division and the changes in size and shape of the developing Bombyx mori embryo were monitored from just prior to blastoderm formation until the onset of gastrulation. Nuclear invasion of the periphery and subsequent cellularization occurs with a marked anterior to posterior gradient. Consequently, there is no syncytial blastoderm stage in which all nuclei simultaneously rest at the periphery. In addition, the posteriormost blastoderm cells do not form pole cells. Prior to gastrulation, the Bombyx blastoderm cells undergo several unusual types of cell behaviors, indicated in part by the modifications and specializations of their lateral edges. Subsequent to the completion of cellularization, localized regions of mitosis can be found within the germ anlage, but at the stage monitored, just prior to the onset of gastrulation, these regions do not appear to be the equivalent of the Drosophila mitotic domains. The implications of these findings for modeling how segmental fates are established in insects are discussed. This work confirms and extends the work of earlier histological investigations of Bombyx development.