708 DR ALEXANDER BRUCE AND DR JAMES W. DAWSON ON 
he has been able to trace simultaneously the ganglion cell processes passing out as 
naked fibres, which then became covered with cells which have also emigrated from the 
wall of the medullary tube. 
LENHOSSEK (1906) considers the question of the origin of the sheath of Schwann 
cells to be not immediately connected with that of the mode of formation of the 
axis-cylinder. ‘Though a convinced centralist, he derives the Schwann cells, which 
he terms lemnoblasts, from the spinal ganglionic anlage. In the fowl embryo and 
in a very early human embryo he has shown that the cells of the spinal ganglionic 
anlage differentiate into ganglion cells and cells which pass along the nerve root as 
sheath of Schwann cells, but that these latter do not form the fibres of the sensory root. 
The lemnoblasts thus correspond to the glia cells of the central nervous system. 
LENHOSSER’S illustration of the glosso-pharyngeal nerve with its ganglion, corresponding 
to a posterior root ganglion, shows that through the whole extent from the ganglion 
to the medulla the bundle of fibres is entirely non-nucleated but is ensheathed by - 
a single layer of cells, whose nuclei are quite distinct from the surrounding mesenchyme 
elements. Later, cells, proliferated and migrated from the side of the ganglionic 
anlage, penetrate both motor and sensory roots, and future lemnoblasts are 
formed by an independent increase of those already penetrated. He believes that 
the spinal ganglionic anlage provides the sheath of Schwann cells for the whole 
peripheral nervous system including the sympathetic. LenHossEK says that the 
supporters of the cell-chain theory cannot get over two facts: the one, the absolutely 
non-nucleated condition of the white substance of the central nervous system; the 
other, the almost non-nucleated condition of the peripheral nerves in certain stages 
of their development. LernHossEK admits that it is conceivable that under patho- 
logical conditions the sheath of Schwann cells, in virtue of their origin from the 
neural crest, may become nerve builders. 
BeruE (1906), whose work on the regeneration of nerves we shall refer to later, 
endeavours to answer LENHOSSEK in the following terms: The first anlage of the 
peripheral nerve consists only of a cell syncytium with nuclei arranged as a border; 
later, the nuclei of this syncytium proliferate and penetrate the protoplasm, giving 
rise to cells arranged in rows, and in the protoplasm of these cells the first axis- 
cyclinders form. BrTHe argues that the developing nerve is just as little non- 
nucleated as the cylindrical epithelium of many gland tubes, and that LenHossex and 
others have used methods which revealed the axis-cylinders but not the development 
of them within the cells. 
Hep (1906).—No review of the work on the development of nerves can afford 
to ignore HeELp’s important memoir and later papers founded on an exhaustive 
investigation of the embryos of trout, shark, frog, rabbit, ete. It is almost impossible 
to give an abstract of his views. They are so far a modification of Hensen’s that 
they are referred to as the Hensen-Hexp hypothesis, which may be stated thus :— 
(1) the cells related to an embryonic nerve path are (a) neuroblasts of His, which 
