<P> In plants, the zygote may be polyploid if fertilization occurs between meiotically unreduced gametes . </P> <P> In land plants, the zygote is formed within a chamber called the archegonium . In seedless plants, the archegonium is usually flask - shaped, with a long hollow neck through which the sperm cell enters . As the zygote divides and grows, it does so inside the archegonium . </P> <P> In human fertilization, a release ovum (a haploid secondary oocyte with replicate chromosome copies) and a haploid sperm cell (male gamete)--combine to form a single 2n diploid cell called the zygote . Once the single sperm enters the oocyte, it completes the division of the second meiosis forming a haploid daughter with only 23 chromosomes, almost all of the cytoplasm, and the sperm in its own pronucleus . The other product of meiosis is the second polar body with only chromosomes but no ability to replicate or survive . In the fertilized daughter, DNA is then replicated in the two separate pronuclei derived from the sperm and ovum, making the zygote's chromosome number temporarily 4n diploid . After approximately 30 hours from the time of fertilization, fusion of the pronuclei and immediate mitotic division produce two 2n diploid daughter cells called blastomeres . </P> <P> Between the stages of fertilization and implantation, the developing human is a preimplantation conceptus . There is some dispute about whether this conceptus should no longer be referred to as an embryo, but should now be referred to as an proembryo, which is terminology that traditionally has been used to refer to plant life . Some ethicist and legal scholars make the argument that it is incorrect to call the conceptus an embryo, because it will later differentiate into both intraembryonic and extraembryonic tissues, and can even split to produce multiple embryos (identical twins), while others have pointed out that, as so - called extraembryonic tissues are really parts of the embryo's body that are no longer utilized after birth, much as milk teeth fall out after childhood, and as the process of the embryo splitting to form identical twins leaves the original intact, while generating a new embryo, rendering it no different from the process of cloning an adult human . However, the National Institutes of Health has made the determination that the traditional classification of pre-implantation embryo is still correct . </P>

When does the fertilized ovum divide after fusion