1. 1. The subcommissural organ (SCO), present in all vertebrates, is situated in the roof of the third brain ventricle, and secretes into this ventricle a glycoproteinaceous, fibre-like structure, the liquor fibe (LF).\ud \ud 2. 2. The three forms of European green frogs diner from each other in the three parameters used to measure the secretory activity of the SCO: the amounts within the SCO of stained secretory material and of secretory material labelled by a radioactive precursor, and the growth rate of LF.\ud \ud 3. 3. The observed differences indicate that the secretory activity of the SCO is distinctly higher in Rana ridibunda than in Rana lessonae, whereas in Rana esculenta this activity is intermediate.\ud \ud 4. 4. These findings confirm the hybrid character of esculenta, which is in many morphological, physiological and ecological features intermediate between lessonae and ridibund
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