Concerning the mechanism of hydrocephalus in the progeny of Trypan Blue treated rats.

1. Seven foetal rats aged 19-21 days were examined in serial section to investigate the lesion responsible for the hydrocephalus in the offsprings of Trypan Blue injected mothers. 2. In four, lesions of the aqueduct were found. Three were examples of atresia which extended into the third ventricle. The fourth showed stenosis. 3. The obstruction in one of the remainder was probably due to impaction of the cerebellum and medulla oblongata in the foramen magnum associated with a spinal myelocele. 4. No satisfactory explanation for the hydrocephalus could be offered in the other two animals.