The blastula is usually a spherical layer of cells (the blastoderm) surrounding a fluid-filled or yolk-filled cavity (the blastocoel). From here the spatial arrangement of blastomeres can follow various patterns, due to different planes of cleavage, in various organisms:Ĭleavage patterns followed by holoblastic and meroblastic eggs HoloblasticĪfter the cleavage has produced over 100 cells, the embryo is called a blastula. In holoblastic eggs the first cleavage always occurs along the vegetal-animal axis of the egg, the second cleavage is perpendicular to the first.
Because cleavage is impeded in the vegetal pole, there is a very uneven distribution and size of cells, being more and bigger at the animal pole of the zygote. On the other hand, meroblastic cleavage occurs in animals whose eggs have more yolk i.e. Holoblastic cleavage occurs in animals with little yolk in their eggs, such as humans and other mammals who receive nourishment as embryos from the mother, via placenta or milk. Depending mostly on the amount of yolk in the egg, the cleavage can be holoblastic (total) or meroblastic (partial). The different cells derived from cleavage, up to the blastula stage, are called blastomeres. The zygote undergoes rapid cell divisions with no significant growth, producing a cluster of cells that is the same size as the original zygote.
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