[Current concepts of the biology of the development of the human head. Observations and hypotheses].

Morphological dynamics controlling vertebrate embryo development, which takes place in flotation in a liquid medium, leads to the hypothesis that the pentameric symmetry of echinodermes, ancestors of the vertebrates, is conserved during evolution. This embryogenesis consists of morphological repetitions: synchronous growth of five expansions specialized in exploration and spatial deplacements (head and four limbs); each of these five expansions consists of five buds or five radii (facial buds, fingers and toes). Homomorphism of the mouth, of the hands, and of the feet--which are capture catastrophes over the environment, installed early during development--allows later functional and sensory connections. These morphological repetitions are also demonstrated by the association of facial, hand, and foot buds, malformations for which a topological correspondance can be shown. During vertebrate morphogenesis by flotation in a liquid medium, other phenomena take place. For instance, the palate is formed anteriorly by a triple junction resulting from the fusion of its three constitutive buds.