1. The brains of chick embryos, of cats six weeks old, of rabbits two months old, and of dogs three weeks old, when cultivated in vitro, develop long filaments which, according to their growth and their anatomical and tinctorial characters, must be considered as true axis cylinders.
2. Similar structures develop from spinal ganglia of rabbits seven months old, and from the spinal cord of cats six weeks old, and of rabbits two months old.
3. When severed from their origin by section these threads undergo degenerative changes which do not appear after nine hours, but which are seen after twenty hours, and continue until in the course of the following two days the thread degenerates completely.
4. After twenty hours the development of new axis cylinders from the central part of the cut fibers is observed.