Observations on the fate of nucleus superficialis magnocellularis of Rendahl in the avian diencephalon, bearing on the organization and nomenclature of neighboring retinorecipient nuclei.

The cytoarchitectonic development of an. superficialis magnocellularis (dorsal thalamus, posterior parencephalon) was studied from 3 days of incubation up to the mature state after hatching in the chick. A hypothesis of Kuhlenbeck (1937) on a partial transformation or contribution of SM cells into a different neighbouring griseum was tested, in the wider context of divergent interpretations of diencephalic development either within Herrick's (1910) longitudinal columnar theory, or within a modified neuromeric conception (Puelles et al. 1987 a). Nucleus SM develops early within the alar region of the posterior parencephalon, forming an outer mantle stratum over the main telencephalopetal thalamic inner cell mass. Thymidine-labeling data pinpoint its generation period mainly between 3 and 4.5 days of incubation. Throughout its subsequent development, SM remains within the primary interneuromeric limits that separate it from ventral thalamus and pretectum. After 8 days of incubation, SM subdivides into superficial (compact) and deep (disperse) sublaminae. The superficial one becomes much compressed between n. geniculatus ventralis and n. synencephali superficialis. Some of its cells migrate interstitially into the optic tract (12-16 days in ovo) and later disappear. The corresponding mature remnant was called n. interstitialis tractus opticus (ITO). The deep sublamina of SM forms a cap around n. rotundus. It becomes increasingly dispersed due to many passing fibers, and may be recognized in the mature brain as an area perirotundica (ApR). Clarification of the fate of embryonic SM bears on the confused terminology for various visual diencephalic nuclei. It is argued that the terms n. geniculatus dorsalis p. principalis and p. intercalaris, n. superficialis magnocellularis (in its wrong usage), n. lamminaris precommissuralis, n. lentiformis mesencephali p. medialis, p. parvocellularis and p. magnocellularis should be considered obsolete, on various embryological and hodologic grounds. An embryologically consistent terminology is proposed.