Basic Planes of Cleavage

The basic planes along which the egg and its daughter blastomeres are divided during early cleavage are:

A. Meridional Plane - the cleavage furrow passes from the animal pole to the vegetal pole through the centre of the spherical egg or the blastomeres so as to divide the egg into two equal halves, e.g., first cleavage furrow in the chick and first as well as second cleavage furrows in the frog's egg.

B. Vertical Plane - the cleavage furrow forms parallel to animal-vegetal axis but a little away from the centre of the blastomeres, e.g., thud cleavage plane of chick blastoderm.

C. Equatorial Plane - the cleavage furrow bisects the egg at right angle to the median axis exactly half way between the animal and vegetal poles the cleavage furrow appears along the equator of the spherical egg, e.g. the third cleavage plane of sea urchin.

D. Latitudinal or transverse or horizontal plane - it is like equatorial but the cleavage furrow passes through the egg cytoplasm on either side of equator along the latitudes of the egg sphere, e.g., third cleavage plane of amphibian eggs.