<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> <P> After fertilization, the conceptus travels down the oviduct towards the uterus while continuing to divide mitotically without actually increasing in size, in a process called cleavage . After four divisions, the conceptus consists of 16 blastomeres, and it is known as the morula . Through the processes of compaction, cell division, and blastulation, the conceptus takes the form of the blastocyst by the fifth day of development, just as it approaches the site of implantation . When the blastocyst hatches from the zona pellucida, it can implant in the endometrial lining of the uterus and begin the embryonic stage of development . </P> <P> The human zygote has been genetically edited in experiments designed to cure inherited diseases . </P> <P> A Chlamydomonas zygote contains chloroplast DNA (cpDNA) from both parents; such cells are generally rare, since normally cpDNA is inherited uniparentally from the mt+ mating type parent . These rare biparental zygotes allowed mapping of chloroplast genes by recombination . </P>

Where does fertilization and zygote formation take place