248 Growth and Histogenesis of Nerves 
that the fibrils may increase in size and multiply by dividing longitudi- 
nally somewhat in the manner which Heidenhain has described for the 
fibrils of muscle-cells. When a nerve branches certain of the fibrils 
belonging to a given axis-cylinder process may be diverted into the 
branch while others may continue in the main trunk. Mays showed that 
in the frog a given nerve-fibre gives rise to a large number of peripheral 
branches. Dunn has recently shown that in the sciatic nerve of the frog 
there is a constant increase in the number of fibres as the periphery 1s 
approached. Although there is at the same time a decrease in area of 
eross section of the individual fibres, this is more than offset by the in- 
crease in number of fibres. All this goes to show that the fibrils con- 
nected with a given ganglion cell must increase very greatly in number 
and mass as one passes from the central toward the peripheral areas. 
The growth and longitudinal division of fibrils therefore probably con- 
siantly increases in amount as the fibrils extend outwards. The fibrils 
of embryonic nerves seem to be larger than the “ primitive” fibrils de- 
scribed by Apathy and by Bethe in adult nerves. The relations of the 
latter to the former can only be determined when methods have been 
devised which will stain primitive fibrils in young embryos. 
The relations of the fibrils to the sheath-cells are difficult to determine 
in very small peripheral branches. The sheath-cells are so closely ap- 
plied to the fibrils that it becomes mainly a matter of judgment to 
decide whether the fibrils are surrounded by or are embedded within the 
sheath-cells. In the large nerve-trunks first formed near the central 
nervous system sheath-cells appear scattered within the nerve as well as 
about the periphery (Fig. 2), and this, too, may lead to dispute as to 
the relation of the cells within the nerve to the nerve-fibrils. It is in 
peripheral nerves of a moderate size found during the earlier stages of 
embryonic development that the absence of genetic continuity between 
nerve-fibrils and the intrinsic cells of the nerve becomes most clear. 
Thus in Fig. 12 is shown a cross section of the median branch of the 
dorsal division of a thoracic nerve of a pig embryo 14 mm. long. Some 
of the surrounding mesenchyme is also shown. This nerve is composed 
of a large bundle of fibrils surrounded by a single layer of flat anas- 
tomosing cells. No cells are to be found among the fibrils, although the 
nerve may be followed for a considerable distance through a series of 
sections. 
After a nerve of this kind has become considerably distended by in- 
growth of new fibrils from behind, cells begin to wander from the in- 
vesting sheath in among the fibrils. These cells give rise through anas- 
tomosis of their membranous processes to a skeletal framework similar 
