Cell division during the early stages of the embryo’s development after fertilisation is known as cleavage. The cleavage can be either holoblastic (complete) or meroblastic (incomplete cleavage), depending on the amount of yolk in the egg.
Similar to echinoderms, cleavage in frog embryos is generally radially symmetrical and holoblastic. The frog egg is mesolecithal in nature and has a lot of yolk. The animal pole and the vegetal pole are located in opposite hemispheres of the egg.
The yolk is more concentrated at the vegetal pole and the nucleus is centred in the animal pole.
Because there is a moderate amount of yolk and unequal distribution of yolk present in the frog egg, the cleavage is of the holoblastic unequal type. This serves as a barrier during cell division.
The first division slowly descends into the vegetal zone from the animal pole.
The second cleavage will have already begun close to the animal pole, while the first cleavage furrow continues cleaving the yolky cytoplasm of the vegetal hemisphere. This cleavage is meridional and at a right angle to the first one.
The third cleavage is equatorial. The cleavage furrow in frog eggs is displaced towards the animal pole due to the vegetally positioned yolk and not at the equator. It splits the frog embryo into four micromeres (or small animal blastomeres) and four macromeres (or large blastomeres), in the vegetal region.
Read Also:
- Difference between Radial and Spiral Cleavage
- Difference between Cleavage and Mitosis
- Cleavage Biology MCQs for NEET 2021
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