If there is only little yolk (oligolecithal eggs), the yolk granules follow the movements of the cytoplasm and are distributed in the resulting blastomeres. This pattern is typical of many animal groups however, more complicated patterns of cleavage are found in such animals as annelids, mollusks, and nematodes.Īs the amount of yolk in the egg increases, it influences cleavage by hindering the cytoplasmic movements involved in mitosis. Subsequent divisions may alternate between vertical and horizontal cleavage planes, but later cleavage divisions become randomly oriented. The third cleavage plane is at right angles to both the first and second cleavage planes and is horizontal, or equatorial. The second cleavage plane is again vertical but at right angles to the first, giving rise to four equal cells arranged around the main axis of the egg. This cleavage plane is arbitrarily called vertical, on the assumption that the main axis of the egg is vertical. As cleavage progresses, the cells no longer divide at the same time.Ĭleavage in most animals follows an orderly pattern, with the first division being in the plane of the main axis of the egg. At the beginning of cleavage, cell divisions tend to occur at the same time in all blastomeres, and the number of cells is doubled at each division. During cleavage, almost no growth occurs between consecutive divisions, and the total volume of living matter does not change substantially as a consequence, the size of the cells is reduced by almost half at each division. The divisions are mitotic- i.e., each chromosome in the nucleus splits into two daughter chromosomes, so that the two daughter blastomeres retain the diploid number of chromosomes. Cells produced during cleavage are called blastomeres.
#BLASTOPORE SERIES#
This task is accomplished by cleavage, a series of consecutive cell divisions. Since the goal of development is the production of a multicellular organism, many cells must be produced from the single-celled zygote.
#BLASTOPORE HOW TO#
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