Without the micromeres, subsequent gastrulation fails. They will give rise to the skeletal elements called spicules in the adult urchin. Drosophila and related insects . Subsequently, complete divisions occur, but they still only partition the peripheral layer of cortex and cytoplasm around the cytoplasmic core. Zebra fish .
Cleavage is superficial, at one pole of the large yolky cytoplasm.
We will consider mammalian early embryos in more detail later. In vertebrates, the first several cleavages result in a tightly packed ball of cells called a morula . The outer cells of the morula become tightly coupled by tight junctions so that ions and small molecules cannot leak to the outside. The mature eggs of most non-mammalian species contain large amounts of protein and RNA synthesized maternally during oogenesis. A third change is that cells become motile and soon initiate the movements of gastrulation.