The embryonic period ranges from the fertilization up to the 8th week of development.
The fetal period comprises that from the 9th week (3rd month) up to delivery and thus lasts much longer.
The embryonic period is divided into a preembryonic period (from the 1rst to the 3rd week) 1-8, which includes fertilization, the implantation and the formation of the bi- and tri-laminar embryo, and into the true embryonic period (3rd to the 8th week) 9-23. During the embryonic period specific tissues and organs arise from each of the three germinal layers. It is the time period of organogenesis in which the embryo takes on its human form and increases its weight from 1/1'000th of a mg to 2-3 g (an increase of 2-3'000'000!). (1)
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