Human development begins when an oocyte (ovum) from a female is fertilized by a sperm (spermatozoon) from a male. … Embryology is concerned with the origin and development of a human being from a zygote to birth. ...
Zygote. This cell, formed by the union of an oocyte and a sperm, is the beginning of a new human being.
Embryo. This term refers to the developing human during all of its early stages of development (e.g., zygote, morula, blastocyst).
Fetus. After the embryonic period (8 weeks), the developing human is called a fetus. During the fetal period (ninth week to birth), differentiation and growth of the tissues and organs formed during the embryonic period occur. Functional maturation of the organs and the rate of body growth are remarkable, especially during the third and fourth months, and weight gain is phenomenal during the terminal months.
Trimester. This period consists of 3 calendar months. Obstetricians commonly divide the 9-month gestation period into three trimesters. The most critical stages of development occur during the first trimester, when embryonic and early fetal development is occurring.
Friday, May 27, 2011
From Before We Are Born: Essentials of Embryology and Birth Defects, 7th edition (2007), by Keith L. Moore and T.V.N. Persaud: