[Rostral curvature of the proximal visceral segment--a previously unknown embryonal movement]

Before the development of the chondrocranium within the head region of the human embryo the following different, so far unknown, tissue shifts are demonstrable: Because of the dorsal brain- sheats of the rostral regions of the brain (mes- and diencephalon) don't have yet blood-vessels (area avasculosa ), they are unable to increase and have to be completed by material from more caudal regions. Therefore from the middle of the 5th up to the end of the 7th week of the embryonic life the mesenchymal and ectodermal rhombencephalic brain- sheats are drawn from caudal to rostral. This becomes possible because the rhombencephalon by that passively is put into the pontine and the neck flexures, which also shift rostralwards , thus allowing the hindbrain to shorten. The proximal ends of the visceral arches I and II, where the anlagen of the temporomandibular joint and of the ear are localized, are parts of the latero-basal rhombencephalic brain- sheats . They take part in of this rostral shift. By that all their axial structures, the first branchial groove and the anlage of the auricle synchronistically undergo positional changes proving the rostral bend of the proximal region of the two visceral arches.