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.

The cleavage in the amphioxus egg is of the holoblastic type, and the egg is isolecithal. The egg is divided into two blastomeres around one hour after fertilisation. The egg axis is traversed from pole to pole by the plane of this first cleavage.

The next cleavage occurs on a vertical plane perpendicular to the initial plane.

The planes of separation in the third phase of division are horizontal. By the 16 to 32-cell stage, the zona pellucida is filled with a dense mass of cells that will have grown close to one another. The embryo is now called a morula.

Blastomeres quickly arrange themselves in a circular arrangement, forming a blastula centred around the blastocoel. The cleavage phase comes to an end at this point.

Read Also:

• Difference between Radial and Spiral Cleavage
• Difference between Cleavage and Mitosis
• Cleavage Biology MCQs for NEET 2021

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