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The series of mitotic cell divisions that a zygote undergoes while passing from the fallopian tubes into the uterus is called as cleavage.

The cleavage is holoblastic because the complete zygote divides. 

It takes place as follows:

  • 1st Cleavage: The plane of first mitotic division is vertical and passes through the center of the zygote. It divides the zygote into 2 equal sized cells called blastomeres, each containing equal cytoplasmic volume and chromosome number. First cleavage is prolonged and takes place 30 hours after fertilization.
  • 2nd Cleavage: The plane of cleavage passes at right angle to the first one, thus forming 4 blastomeres. After the cleavage, sometimes one of the blastomeres divides earlier than the second one forming the 3-cell stage. Then, the second blastomere divides forming the 4-celled stage.
  • 3rd Cleavage: The four blastomeres divide to form 8 blastomeres.
  • 4th Cleavage:  The 8 blastomeres again divide to form 16 blastomeres. The 16-cell stage is spherical and resembles a mulberry in structure. Hence, it is called as morula.
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