Known as the paraphysis cyst, or the colloid cyst of the third ventricle, the lesion described in this study continues to have an unprecise etiology in spite of a great number of investigations. This series of 75 anatomo-clinical cases with this lesion provides sufficient material for a valuable statistical analysis. After enumerating the different cerebral symptoms of this disease, the author makes a macroscopic and microscopic morphological analysis and compares his own results with those found in literature. The embryologic investigations mentioned and the hypotheses to which they lead do not shed light on the origin of these cysts. The mathematical analysis indicates a difference in frequency between both sexes, with a far greater frequency in males. Referring to the grey commissure's agenesia, mentioned by Morel as statistically more frequent in men than in women, the author concludes that the colloid cysts of the third ventricle are also to be considered as a cerebral anomaly related to sex.