CBSE Class 12-science Answered

As the human zygote passes through the fallopian tube to the uterus, cleavage occurs. The cleavage is holoblastic and it divides the blastomeres completely into daughter cells. The first cleavage takes place about 30 hours after fertilization. It is meridional, coinciding with the animal-vegetal pole axis. It produces two blastomeres, one slightly larger than the other. The second cleavage occurs within 60 hours after fertilization. It is at right angles to the plane of the first, and divides each blastomere into two by forming a mitotic spindle in each. Third cleavage takes place about 72 hours after fertilization. Subsequent cleavage divisions follow one after another in an orderly manner, but in a less precise orientation. Cleavage produces a solid ball of small blastomeres. The embryo now looks like a mulberry, and is known as morula. It refers to the lobed surface of the embryo at this stage.