<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 released 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). Others have pointed out that so - called extraembryonic tissues are really part of the embryo's body that are no longer used after birth (much as milk teeth fall out after childhood). Further, as the process of the embryo splits to form identical twins--leaving the original tissues intact--a new embryo is generated, 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>

N all plants the zygote and earliest stages of the developing embryo are