Dictionary

Imaginal rudiment - formation through an invagination from the ectoderm that fuses with the middle sac. This rudiment develops a fivefold symmetry, and skeletogenic mesenchyme cells enter the rudiment to synthesize the first skeletal plates of the shell.

Cetrosome-attracting body (CAB) - a macroscopic subcellular structure that positions the centrosomes asymmetrically in the cell.

Energids - nuclei and their associated cytoplasmic islands.

Notochord - a transient mesodermal rod that plays an important role in inducing and patterning the nervous system.

SKN-1 protein - a maternally expressed polypeptide that may control the fate of the EMS blastomere, which is the cell that generates the posterios pharnx.

P-granules - ribonucleoprotein complexes that specify the germ cells.

Invagination - The infolding of a region of cells, much like the indenting of a soft rubber ball when it is poked.

Involution - The inturning or inward movement of an expanding outer layer so that it spreads over the internal surface of the remaining external cells.

Ingression - The mirgration of individual cells from the surface layer into the interior of the embryo. The cells become mesenchymal and migrate independently.

Delamination - The splitting of one cellular sheet into two more or less parallel sheets. While on a cellular basis it resembles ingession, the result is the formation of a new sheet of cells.

Epiboly - The movement of epithelial sheets that spread as a unit to enclose the deeper layers of the embryo. Epiboly ca occur by the cells dividing, by the cells changing their shape, or by several layers of cells intercalating into fewer layers. Often, all three mechanisms are used.