Charles Russell Bardeen 255 
leave the central nervous system and spinal ganglia in naked bundles 
(Fig. 1) but soon become intimately related with sheath-cells which ac- 
company them closely throughout the period of growth. At the growing 
tip of a nerve it is difficult to decide whether the axis-cylinder fibrils 
or the sheath-cells proceed (Figs. 2, 4 and 9). Posterior to this the 
nerve gradually becomes distended with fibrils by ingrowth from behind 
and by multiplication due to division. In an early embryonic nerve 
of moderate size one finds many hundred fibrils inclosed by a sheath of 
flattened cells, but with no cells among them (Fig. 12). In such nerves 
one can most easily see that the fibrils are not differentiated parts of cells 
lying in the nerve. In addition to this it is possible to isolate from em- 
bryos one to five or six centimeters in length unensheathed nerve-fibril 
bundles from one-half to a millimeter long. When the sheath of Schwann 
is formed in embryos ten to twenty centimeters in length, the nuclei 
of the sheatheare about a tenth of a millimeter apart. There is, there- 
fore, no segmentation in the axis-cylinder fibrils corresponding to the 
cells of the sheath of Schwann. 
Union of nerve and muscle fibres takes place before the forma- 
tion of the sarcolemma. he latter membrane becomes so closely united 
to the sheath of Schwann that no line of demarcation can be seen be- 
tween them in specimens from which the muscle substance has been 
digested. The terminal apparatus of the nerve is more resistent to diges- 
tive fluids than the muscle substance, and is closely attached to the under 
surface of the sarcolemma (Fig. i1). 
The sheaths of the nerves serve to maintain the stroma in which 
the axis-cylinder fibrils grow. At first large numbers of fibrils are en- 
sheathed within the main trunks of the nerves, but by proliferation of 
sheath-cells smaller and smaller bundles are inclosed until finally but a 
small group of fibrils is inclosed within each sheath of Schwann. The 
work of Gurwitsch on the formation of the sheath of Schwann is con- 
firmed. Myelinization is due to influences exerted by the axis-cylinder 
fibrils on the surrounding stroma. 
During development the nerve funiculi may be broken up into smaller 
funiculi by invasion of tissue of the investing sheath. 
LITERATURE CITED. 
APATHY, S.—Das leitende Element des Nervensystems, ete. Mittheilungen 
aus der zoolog. Station zu Neapel, XII, 495, 1897. 
BARDEEN, C. R.—The development of the musculature of the body-wall in the 
pig, including its histogenesis and its relations to the myotomes 
and to the skeletal and nervous apparatus. The Johns Hopkins 
Hospital Reports, IX, 367, 1900. 
