The reflex nature of progression has been the subject of a large number of investigations, and it is generally admitted that the thalamic cat and the midbrain rabbit exhibit fairly normal progression. Progression, as exhibited by the thalamic cat, consists of at least three components, which must be properly synthesized and coordinated for walking to occur: (1) rhythmicity, which is known to be present, for the hind limbs, at least, in spinal animals; (2) static or postural tonus, which is present, according to some observers, in the spinal animal and which is much more pronounced in the decerebrate animal with transection at the level of the posterior colliculi, and (3) equilibration and maintenance of upright posture, for which the rostral portion of the tegmentum of the mesencephalon or its continuation into the hypothalamus is necessary. The decerebrate animal prepared by a transection through the mesencephalon possesses the first two of