The Localization of the Median Plane of the Embryo

The eggs of many animals appear under the microscope to be radially symmetrical around a primary axis. This axis is an imaginary line extending from that region on the surface at which the polar bodies are given off (pole) through the center of the egg to the opposite pole (antipole). Sometimes the pole is indicated by the distribution of pigment or by some structure at the surface. The materials of the egg are stratified or graded from the pole to the antipole.