The process of Wallerian degeneration of myelinated nerve fibers in the pyramidal tract of the rhesus monkey.

1. The process of Wallerian degeneration was investigated in the pyramidal tract of rhesus monkeys which had experimental lesions in the brain and had survived for a variety of periods, from 3 to 787 days, after physiological and neurological studies of the pyramidal tract lesions. 2. The nervous tissue samples were embedded in Epon 812 and observed by electron microscopy of ultra-thin sections and by light microscopy of semi-thin sections. In this way, it was shown that both normal and degenerated nerve fibers could be clearly identified, even by light microscopy. 3. The thick fibers of the pyramidal tract were attacked and engulfed by phagocytes at an early stage of 7 days after operation in such a manner that the phagocytes invaded the myelin tubes via the nodes of Ranvier, and engulfed the fragmented axons and myelin lamellae which had been peeled off from the paranodal regions. However, these phagocytes soon disappeared from the degenerated areas. Phagocytes then reappeared rapidly among the nerve fibers about 9 weeks later, and intensively phagocytosed the degenerated myelin sheaths which had not been consumed at the early stage, converting them into lipid droplets. These phagocytes may be more reasonably considered as reactive microglia, or if not so, exogenous macrophages. 4. The thin fibers were degenerated slowly and disappeared probably by phagocytosis and by an autolytic process.