A 4-month-old infant has undergone surgical treatment for meningomyeloencephalocele. At bih, an operation was carried out in the posterior cranial fossa to paially replace brain cerebellar contents to an intracranial position. In investigations for progressive hydrocephalus, it is noted that there is herniation of the cerebellar tonsils through the foramen magnum, and a diagnosis of Arnold-Chiari syndrome is established. This syndrome may also include which of the following?