[Embryonal development of the brain of the shark Scyliorhinus canicula (L.). I. Formation of the shape of the brain, the migration mode and phase and the structure of the diencephalon].

Brain structure of the shark Scyliorhinus canicula has been studied on the basis of a series of embryonic stages. The incubation time was 157 days in average (under constant conditions of 15 centigrades and density of 1.026). 1. The first predominant characters of the external brain shape are the optic tectum, cerebellum, telencephali hemispheres and oculomotor root. In addition, the brain axis is characterized by new flexures in the cerebellar and prosencephalic level. A nuchal flexure in absent. 2. The ontogenetic pattern of the matrix layer differs in different brain parts. Generally, ventral columns develop earlier than dorsal ones. Matrix development and cell proliferation occur especially late in the cerebellum. For the most part the results agree with findings by KAHLE (1951) concerning the human brain. The telencephalon is an exception and shows a developmental retardation, while a regular development is observed in Scyliorhinus. 3. The hypothalamus overlaps the other regions of the diencephalon. The ventral thalamus is small, a dorsal thalamus is not observed. The pretectal region is remarkably differentiated and contains a rich set of nuclei. 4. The epithalamus is asymmetrical; a phenomenon that occurs after hatch. The left habenula is distinctly larger than the right-one.