252 Growth and Histogenesis of Nerves 
the spinal ganglia and cord along with the bundles of axis-cylinder pro- 
cesses. These cells may contribute in part to the origin of the cells of 
the sheath of Schwann, but my observations lead me to believe with 
Vignal and Gurwitsch, that the latter arise in mammals, in the main at 
least, from the mesenchyme. I agree with Gurwitsch that the sheath 
of Schwann is an exogenous structure which has quite a different genetic 
origin from that of the nerve-fibre. When once the sheath of Schwann 
is formed it does, however, undoubtedly enter into intimate physio- 
logical relations, “ symbiosis,’ with the axis-cylinder process which it 
incloses. 
Indeed, throughout the period of the development of the nerve-fibrils 
as well as in the adult life, the elaboration of nerve-sheaths shows that 
they must play a vital part. In the embryo, at least, their function is 
doubtless mainly nutritional. The early nerves are composed of nerve- 
fibrils within a sheath of anastomosing cells. In addition to the fibrils 
there is contained within the sheath some substance either fluid or semi- 
fluid in nature, which serves to “ float” the growing fibrils and to fur- 
nish them with nutrition. Vignal has described this substance as homo- 
geneous. It deserves much more careful study from the micro-chemical 
standpoint than it has yet received. The perineural sheath in nerves 
before the sheath of Schwann has appeared has, in all likelihood, specific 
action in determining the physical and probably also the chemical char- 
acteristics of this stroma. 
For the formation of a medullary sheath a special cellular sheath 
about each nerve fibre is not absolutely necessary. As Gurwitsch has 
pointed out, formation of myelin about an axis-cylinder process may 
begin before the corresponding sheath of Schwann is complete. I have 
observed this also in the pig. Kolster in a valuable study on the devel- 
opment of the nerve-fibres in Salmo trutta has shown that a medullary 
sheath may develop about fibres in a nerve in which no cells have as yet 
passed from the perineurium into the midst of the fibres which it sur- 
rounds. Apathy has paid special attention to the interfibrillar sub= 
stance of nerve-fibres and has shown that it may present many charac- 
teristics of myelin. 
Asa rule, however, in mammals the sheath of Schwann, as shown by 
Vignal, completely incloses the axis-cylinder fibrils before formation of 
the medullary sheath begins. In Fig. 15, a, is shown a fibre of this kind 
dissected from the intercostal nerve of a pig 15 cm. long. The cells com- 
prising the sheath here shown are nearer together than is common at the 
period when the sheath is first formed. The average distance between 
nuclei at the period under discussion seems to be about a tenth of a milli- 
