The single cell formed at conception from the union of the sperm and the ovum
Germinal Period
The period that begins at conception and lasts until the developing organism becomes attached to the wall of the uterus about 8 to 10 days later
Embryonic Period
The period that extends from the time the organism becomes attached to the uterus until the end of the 8th week of pregnancy, when all the major organs have taken primitive shape
Fetal Period
This period begins the 9th week after conception, with the first signs of the hardening of bones, and continues until birth
Cleavage
The series of mitotic cell divisions that transform the zygote into the blastocyst
Heterochrony
Variability in the rates of development of different parts of the organism
Heterogeneity
Variability in the levels of development of different parts of the organism at a given time