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During fertilization, the nuclear membrane of the pro-nucleus of the ovum and sperm degenerate. Is the cell is stage called a zygote? After the dissolution, mitosis occurs and two cells are formed.Or is the cell is stage called a zygote? I'm confused as i knew a zygote was single-celled.

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A soon as fertilization occur, the "thing" is called a zygote. It means that a zygote can be a single cell. I don't know where are your difficulties but you might appreciate checking the definitions of: Morula, Blastual, gamete, gametophyte, spermatophyte, plasmogamy, karyogamy. You will easily find all these informations on wikipedia. Did I answer your question ? Where you thinking about a specific species that have an unusual reproctive system like some fungi ? –  Remi.b Jul 27 '13 at 21:49
    
Oups.... did I write can be a single cell. I'm sorry. It is a single cell. After division, it is no more a zygote. –  Remi.b Jul 27 '13 at 22:07
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Conventionally, a zygote is considered to be formed the moment that a spermatozoum, penetrates the cell membrane of the ovum and yields its genetic material into the ovum. Effectually, however, there is a lag between the instant of fertilization and the fusion of the male and female pronuclei. In mammals, the duration of this lag period is ~12 hours. There are also additional actions that must be completed before the first mitosis as in most mammals, including humans, the ovum is actually in the second metaphase of meiosis at the time of fertilization.

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