Development of mesencephalic nuclei in the chick embryo (Gallus domesticus).

Abstract

1. The mesencephalic borders in the chick have been analysed and reconstructed. It is stated that the di-mesencephalic border corresponds to the border between postneuromeres 5 and 6, and the mes-rhombencephalic border with the border between postneuromeres 7 and 8. 2. The migration areas have been carefully analysed and reconstructed. 3. The ontogenesis of the mesencephalic brain nuclei has been traced from migration areas by means of reconstructions of successive series of stages. Some facts may be taken into consideration: a) The pretectal nuclear complex originates from the diencephalic region, close to the di-mesencephalic border, and during its expansion this nuclear complex passes the di-mesencephalic border, so the nuclei come to lie in both the diencephalic and mesencephalic regions. b) Some mesencephalic nuclei originate from a mixed cell material of different migration areas. Brain nuclei from a mixed cell material seem to be: the nucleus mesencephalicus lateralis, the nucleus isthmii, pars principalis complex, the nucleus of the basal optic root, the nuclei of the lateral genicular complex. 4. The mesencephalic anatomical topography is seen to change as follows: The optic tectum starts a rotation caudorostrally; it divides into two separate lobes, which are pushed away laterally along the tegmental sides. The tegmentum expands very greatly and there is tegmental axial straightening. The agueduct comes to lie horizontally.