During cleavages, differnt cleavage furrows may divide the egg from different planes. Few important cleavage planes are following:
1. Meridional plane– When cleavage furrow passes through the centre of animal-vegetal axis and bisects the both poles of the egg, then such plane of cleavage is called meridional plane, e.g., Rana pipiens, Lepidosteus osseus and first cleavage furrow of chick.
2. Vertical Plane– The vertical plane of cleavage is a furrow which tends to pass in a direction from the animal pole toward the vegetal pole. But like meridional plane, it does not pass through the median axis of the egg, but courses to one side of this axis, e.g., third cleavage furrows of Amia calva, Lepidosteus osseus and chick.
3. Equatorial Plane– The equatorial plane of cleavage bisects the egg at right angles to the main axis and half way between the animal and vegetal poles, e.g., fifth cleavage planes of the eggs of Ambystoma maculatum, and first cleavage plane of eggs of higher mammals.
4. Latitudinal Plane– The latitudinal plane of cleavage is similar to the equatorial, but it courses through the cytoplasm on either side of the equatorial plane. It is also called transverse or horizontal plane, e.g., third cleavage planes of Amphioxus and frog.
Determination of plane of cleavage- The plane of cleavage is determined by the position of mitotic spindle of the dividing egg or blastomere, becasue as a rule, the plane of cell division (cleavage) remains always at right angles to the long axis of the spindle. Therefore, the direction of the division plane in refernce to the whole cell is determined by those forces responsible for the mitotic spindle. Ordinarily in a homogeneous (isolecithal) spheroidal egg, the mitotic spindle will be oriented transversely to the polar axis, and thus, the first mitotic division will be vertical or meridional. The spindles for the second set of divisions will then line up parallel to the first division plane, so that the ensuing divisions are again vertical but at right angles to the first, with each quadrant as long as the entire egg, but no more than half its diameter, the spindles for the third set of divisions will now lie parallel to the polar axis, and the divisions themselves will be horizontal and thus at right angles to the previous two. i.e., transverse, horizontal or latitudinal.