Planes of Cleavages
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.
Random Posts
Coral Reef formation and its importance (0)
1. What are corals? Ans. Corals are the anthozoan polyps of Phylum Cnidaria, which are capable of se...What is DNA-helicase? (0)
The unwinding process of two strands of DNA is catalyzed by enzymes called DNA helicases. The major ...
The older animals and humans have some localized growth of tissue, having cells that are closely res...What is SSB protein? (0)
Once the DNA strands are unwound by DNA helicase, they must be kept in single stranded condition. Th...
A large terrestrial or aquatic ecosystem characterized by distinctive kinds of plants and animals an...
Effective pest control may be envisaged as reduction or maintenance of a pest population belong the ...Definition of Parasite, Hyperparasitism, Predator (0)
Parasite: An organism that depends on its host for some essential metabolites and must do some harm...What are the proto-oncogenes? (0)
The cellular homologs of viral oncogenes are called proto-oncogenes or sometimes normal cellular onc...Asexual Reproduction in Paramoecium (0)
This free living protozoa reproduces by asexual and sexual methods. In Paremoecium, the asexual repr...
1. Immortalization of cancer cell: Cancer cells are characterized by immortalization; they overcome ...
