Charles Russell Bardeen 251 
of the cell. After the sheath of Schwann is formed, cell division does not 
take place in its component cells. In regeneration, however, according 
to most observers, indirect division takes place in the cells which arise 
from the cells of the sheath of Schwann. These processes remind one of 
those taking place in muscle-cells. Myoblasts divide by indirect divi- 
sion. The central nuclei of the elongated spindle muscle-fibres multiply 
by direct division and so do also the peripheral nuclei which later appear. 
In regeneration, on the other hand, the myoblasts which arise from the 
muscle-fibres multiply by indirect division. 4 
After they begin to appear the intra- 
funicular cells increase rapidly in num- 
ber and give rise to membranous septa 
which divide the fibrils into smaller and 
smaller bundles and finally surround 
small groups of fibrils with a septum, 
the sheath of Schwann. Hie 13, 5, 
shows the membranous septa of bun- 
dles of fibrils in a pig-embryo 8 em. 
long. Considerable groups of fibrils 
are enclosed. Fig. 14, a, shows a cross- Fia.14. a, Cross-section from the cen- 
ay : » tre ofan intercostal nerve of a pig-em- 
section of a small portion of a nerve of pryo 15 em. long; b, Cross-section of 
an embryo 15 em. long. The septa here rehire eda cag aed ae glee Ta 
form sheaths of Schwann about smal] '"® 2 ¢#™ 
dense bundles of fibrils, although not all the fibrils are thus inclosed. 
Gurwitsch suggests the possibility that the membranous septa which 
he describes might be taken to represent endoneurium. In that case one 
would expect to find two sorts of cells within the funiculus, one set 
belonging more intimately to the nerve-fibrils, the other giving rise to 
the membranous septa. Gurwitsch, however, shows that stage by stage 
the development of membranes within the fasciculus may be followed 
until the sheaths of Schwann appear. A study of a large number of sec- 
tions and teased preparations has served to convince me that Gurwitsch 
is correct in his interpretations. The endoneurium develops compara- 
tively late. It is very slight at the time of the formation of the sheath 
of Schwann. 
There is a vast amount of literature connected with the genetic origin 
of the cells of the sheath of Schwann and of the relations of these cells 
to the axis-cylinder. Those who assume that the latter structure arises 
by fusion of parts derived from a chain of cells, usually consider that the 
cells of the sheath have an ectodermal origin. In the mammals, as well 
as in the lower vertebrates, a certain number of cells wander out from 
