The sex of the embryo is determined by the chromosomal complement of the spermatozoon. This cell results from the union of an oocyte and a sperm. The zygote is characteristic of the last phase of fertilization and is identified by the first cleavage spindle. It is a unicellular embryo. During the first week, the embryo becomes a solid mass of cells and then acquires a cavity, at which time it is known as a blastocyst.
Fertilization takes place in the oviduct.
After the oocyte finishes meiosis, the nuclei of the male and female gametes unite, resulting in the formation of a zygote containing a single diploid nucleus. Embryonic development is considered to begin at this point. This moment of zygote formation may be taken as the beginning or zero time point of embryonic development. Dianne N. Irving has been involved in pro-life activities for many years. She is a former career appointed bench NIH research biochemist.