ABSTRACT
This chapter describes that, even before fertilization, many eggs acquire a regionally differentiated cytoplasm due to the targeted transport and transcription of mRNA molecules. It examines the phases of development known as blastula formation, gastrulation, and neurulation, driven largely by the concerted movements of sheets of cells, and cell migrations in which cells move as individual units. The cellular differences that emerge during embryonic development can frequently be traced back to regional inhomogeneities in the egg cytoplasm. In most nonmammalian species, the egg after fertilization has localized accumulations of specific proteins and mRNAs. The cellular changes that cause neurulation are typical of those occurring throughout development. Treatment of embryos with inhibitors of microtubule polymerization blocks placode formation and subsequent neurulation. The response of a growth cone at any specific location of the embryo will be highly individualistic, so that different growth cones moving over the same terrain can and do select different paths.
