144. Victor E. Emmel. 
fibrillee were not of equal size and were distributed in an interesting 
manner. In some of the preparations, the larger or heavier fibrils 
were mostly at the periphery of the axis cylinder, whereas the finer 
ones were nearer the center (Fig. 29). Consequently at certain 
stages of regeneration the nerve fiber could be structurally analyzed 
into four parts: (a) a central core of very delicate fibrille; (b) a 
peripheral layer, not sharply marked off from (a) but containing 
heavier fibrils; (c) the external sheath of nucleated cells or future 
neurilemmma, and (d) a sheath of cytoplasm between (b) and (ce) 
relatively free from neurofibrille. In later stages of differentiation 
the heavier fibrils predominate (Fig. 30). At the time when the 
neurofibrille are beginning to appear, the cytoplasm of fhe nucleated 
sheath, as compared with that of the axis cylinder, is more coarsely 
granular. In some regions there also appears to be a membrane 
present between the nucleated sheath and axis cylinder. 
No conclusive evidence was obtained as to whether the axis cylin- 
der is formed in situ from the sheath cells, or whether it is an out- 
growth from the axis cylinder of the old nerve cell. In either case, 
however, if the present observation is correct, that the nerve fiber 
develops within a cord of cells proliferated from the ectoderm of the 
various new limb segments, it points to the sheath cells as one of the 
factors involved in determining the final distribution of nerve fibers 
within the regenerated limb. It was not determined whether the 
neurofibrillee differentiate distally or proximally, or whether they 
appear simultaneously throughout the regenerating nerve fiber. The 
fact that in at least certain stages of differentiation the finer fibrille 
occur at he center, and the heavier fibrils at the periphery of the fiber, 
apparently lends support to the conclusion that the neurofibrille are 
being derived from the cytoplasm of the axis cylinder. 
3. Connective Tissuc.—The connective or supporting tissue of the 
crustacean limb consists mostly of broad loose bands or sheets ex- 
tending between opposite walls of the exoskeleton, around the ex- 
tremities of each segment, and in certain regions filling almost the 
entire segment. In addition to these sheets and bands of connective 
tissue there is also a thin layer of simple flat epithelium found cover- 
ing the inner surface of the epidermal cells (here known as “Grenz- 
lamelle”) and surrounding the muscle bundles. 
