They go on to form the cerebellum, which mostly helps coordinate muscular activity, and the brainstem, which plays a vital role in relaying information between the body and the higher regions of the brain.
Its cerebellum, a brain structure that in living birds helps with motor control during flight, was smaller than in today's bird species and more like Archaeopteryx's.
So he closed up his lab for the night, and, as the story goes, in his disappointment, he forgot to remove the slice of someone's cerebellum that he had left sitting in the solution.
These mutations result in degenerative changes in the cerebellum and often in the spinal cord which causes progressive problems with coordination and balance known as ataxia.
So with the brainstem and cerebellum covering your basic life and motor functions, you start to see somewhat more complex tasks being carried out in the next major brain structure, the diencephalon.
This is particularly dangerous because the displaced cerebellum can push onto the brainstem affect neurons responsible for breathing and cardiac function.
In upward herniation, there's a displacement of the cerebellum upward, through a notch in the tentorium cerebelli, while in tonsillar herniation, parts of the cerebellum slip down through an opening in the skull called the foramen magnum.