Published online by Cambridge University Press: 29 April 2021
The traditional notion of the embryo as the developmental phase in which, starting from an undisputable origin (egg, seed) the outline of the bodily architecture of a multicellular organism is shaped, deserves critical discussion. Development does not necessarily have a recognizable starting point. Some of the cells deriving from the zygote may not contribute to the embryo. There are significant differences between the early developmental stages of animals and plants. In animals, at the end of embryonic development the entire structure of the organism is almost always delineated. In plants, the seedling formed during the so-called embryonic development contains only the shoot with the first leaves and the radicle, while the entire structure of the plant, including almost all the leaves and all the flowers, will form from groups of stem cells generated through the entire life of the plant. Development does not necessarily produce an increasing division of labour. Development is not necessarily irreversible. Individual organs are not the products of a distinct developmental process. Differences between species do not always increase progressively from the egg on.