The zygote is formed by the fusion of the spermatozoon and the ovum. This process is known as fertilisation, without which the ovum usually degenerates in 1 day after ovulation. Human cleavage is known as holoblastic because the entire ovum is involved in cleavage. In birds, there is abundant yolk and the yolk part of the egg does not take part in cleavage or cell formation and is thus known as meroblastic.
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