MULTIPLE NEUROMATA OF THE CENTRAL NERVOUS SYSTEM. 707 
nature, of the Schwann cells is deeply rooted ; and even those who admit their ectodermic 
origin, ¢.g. KOLLIKER, LenHosseK, and Harrison, are still pronounced adherents 
of the outgrowth theory, and say that the ectodermic or mesodermic origin of the 
Schwann cells has nothing to do with the question whether the axis-cylinder is an 
outgrowth or not. Koun, however, was not blind to the distinction between the 
problems of the origin of the sheath of Schwann cells and the development of the 
axis-eylinder, but he felt that a clear proof of the ectodermic origin of the cells in 
question would be a very strong argument in favour of their participation in the 
building up of the nerve fibre. 
Koun, studying the development of the dorsal nerve root in the rabbit embryo, found 
that the spinal ganglionic anlage is from the beginning in direct continuity with the 
medullary tube. At first this is only a protoplasmic connection which later develops 
into a cellular stem composed of the very same cells which form the ganglionic anlage. 
‘A further stage is the differentiation of the cells in the ganglion into more and more 
typical ganglion cells, and the cells of the stem into elongated tubes with oval nuclei 
which, as the nerve root lengthens, become more and more the typical cells of the 
sheath of Schwann with a differentiation of their protoplasmic substance into nerve 
fibres. Later on, and at first sporadically, connective tissue penetrates the root. 
Therefore, from the cells of the embryonic anlage arise two types of cells, ganglion 
cells and nerve fibre cells. ‘The Schwann cells of the posterior nerve roots, therefore, 
have not only an ectodermic origin but are of true nervous nature—nerve-forming 
cells. To give the name Scheidenzellen to such elements, Koun considers a 
serious mistake, which has contributed greatly to their want of recognition as nerve 
elements. He suggests the name ‘‘neurocytes”’ for the early undifferentiated cells. 
Koun similarly traces the development of the sympathetic ganglia and its nerves 
to the migration of embryonal undifferentiated neurocyctes to form the sympathetic 
ganglionic anlage. In the anlage these cells can be recognised to differentiate into 
sympathetic ganglion cells and nerve fibre cells and the latter to form nerve tubes. 
Before the characteristic ganglion cells have become differentiated, 7.e. while they 
are a-polar, the early nerve tubes appear as elongated band-like syncytia with numerous 
nuclei. The presence of ganglion cells in relation to cerebro-spinal nerves can thus 
be traced to the migration of embryonal neurocytes. It is thus seen that Koun 
contests the view of the origin of the sympathetic ganglia directly from preformed 
ganglion cells cut off from the distal pole of the spinal ganglia. In the rabbit embryo, 
and also in Selachians, he has traced embryonal neurocytes bending ventralwards 
from the path of the mixed nerve. By their proliferation we get cell accumulations 
which form the anlage of the sympathetic cord, and by their further differentiation 
we get the ganglion cells and the nerve fibres of the sympathetic. The significance 
of this view in relation to the formation of ganglio-neuroma will be discussed later. 
Froriep (1905) admits the ectodermal origin of the sheath of Schwann cells, but 
considers the axis-cylinders to be outgrowths of a central cell. In Selachian embryos 
