Embryonic period usually refers to the period that starts with fertilization ultimately ending eight weeks later. Often, a distinction is made between the blastocyst period and the embryonic period. The period of the blastocyst is taken as the time from conception till the embryonic stages. Thus the embryonic period is defines as starting from three weeks until the conclusion of the 8th week of pregnancy. After this, the foetal stage commences, which continues until childbirth.
The staging of human embryos (the embryonic period being from week three to the end of week eight) was introduced by Franklin P. Mall in 1914 at the Department of Embryology of the Carnegie Institution of Washington. His successor was George L. Streeter, who went on to refine the categorisation of human embryos into 23 stages, or what were called "developmental horizons".
The staging of human embryos (the embryonic period being from week three to the end of week eight) was introduced by Franklin P. Mall in 1914 at the Department of Embryology of the Carnegie Institution of Washington. His successor was George L. Streeter, who went on to refine the categorisation of human embryos into 23 stages, or what were called "developmental horizons".
