<P> Toxic exposures during the germinal stage may cause prenatal death resulting in a miscarriage, but do not cause developmental defects . However, toxic exposures in the embryonic period can be the cause of major congenital malformations, since the precursors of the major organ systems are now developing . </P> <P> Each cell of the preimplantation embryo has the potential to form all of the different cell types in the developing embryo . This cell potency means that some cells can be removed from the preimplantation embryo and the remaining cells will compensate for their absence . This has allowed the development of a technique known as preimplantation genetic diagnosis, whereby a small number of cells from the preimplantation embryo created by IVF, can be removed by biopsy and subjected to genetic diagnosis . This allows embryos that are not affected by defined genetic diseases to be selected and then transferred to the mother's uterus . </P> <P> Sacrococcygeal teratomas, tumours formed from different types of tissue, that can form, are thought to be related to primitive streak remnants, which ordinarily disappear . </P> <P> First arch syndromes are congenital disorders of facial deformities, caused by the failure of neural crest cells to migrate to the first pharyngeal arch . </P>

Sequence the process of the zygote becoming an embryo