714 DR ALEXANDER BRUCE AND DR JAMES W. DAWSON ON 
ring cells represent elements derived from the syncytium and that the protoplasm 
represents endoplasm which is gradually transformed into exoplasm, which in its turn 
is transformed into the lamellated reticulum of the central nerve fibre by a process similar 
to that described in the development of connective tissue fibres. The signet-ring cells, 
therefore, diminish in number and size with the age of the embryo, and later can <— 
be distinguished, even if present, from flattened neuroglia cells. 
BaRILE (1910) records the investigations of PaLuUDINo in trygon violaceus, in . whiee 
he found nuclei in relation to the axis-cylinder within the spinal cord, and also the ex- 
amination of.a teratoma from the neck in which Nios1 found a nodule of the structure 
of the central nervous system, with the axis-cylinders developing in relation to a chain 
of cells. 
(2) Tue Hisrocenesis or NERVE Fipres IN REGENERATION. 
It has long been recognised that the manner of regeneration of any tissue follows 
very closely its first development, z.e. that the first stage of each newly produced ana- 
tomical element is an embryonal one, and this undergoes successive transformations. 
We would thus expect to meet with the same diversity of views regarding the origin of 
the new-formed fibres in regeneration of a nerve after section as we found in regard 
to its embryogenesis. The possibility of connective tissue cells, in virtue of some 
mysterious adaptation, taking over the role of embryonic nerve cells and forming 
new nerve fibres, is too improbable to be discussed. We therefore pass at once to state 
the two main opposing views :— 
(1) The classical teaching is that a budding of the axis-cylinder takes place from the 
last preserved segment of the central end. This corresponds to the central budding 
or outgrowth theory of development, and has also been named the monogenist, or uni- 
cellular, or centralist view. 
(2) The newer teaching is that regeneration takes place by means of the differentia- 
tion of new-born cells which have arisen from the proliferation of the sheath of Schwann 
nuclei. This corresponds to the cell-chain theory of development, and has also been 
named the autogenist, or multicellular, or peripherist view. 
We may here note that though complete agreement has not by any means yet been 
reached, a certain accord, as we shall see later, has been attained. Many centralists 
have yielded the ectodermic origin of the sheath of Schwann cells, thus eliminating the 
most serious objection that mesodermic elements shared in the regeneration of nerve 
fibres; they have also allowed the constant presence of cell-chains in regeneration, 
denying only the actual genesis of the nerve fibres from them. ‘The peripherists, on 
the other hand, whose common standpoint is the conception of the peripheral genesis 
of the nerve fibre, have nearly all conceded that the ultimate differentiation comes only 
after the establishment of a connection with the centre. 
