702 DR ALEXANDER BRUCE AND DR JAMES W. DAWSON ON 
(1) EMBRYOGENESIS. 
Regarding the initial stages of the development of the peripheral nerves, there is 
far from any agreement amongst embryologists. At present three different theories 
hold the field, to which Durante has given the names of central budding, cellular 
lengthening, and cell migration with the formation of cell-chains. Others have called 
them the outgrowth theory, the protoplasmic or intercellular bridge theory, and the 
cell-chain theory. 
1. Theory of Central Budding.—Bipper and Kuprrr, in 1857, put forward the 
view that the peripheral nerves develop by the budding of a central cell, whose. pro- 
longation 
a homogeneous axis-cylinder without nuclei—reaches to the periphery and 
is only secondarily surrounded by mesodermic elements to form its sheath. Hrs, 
KOLLIKER, and others have supported this view. 
2. Theory of Cellular Lengthennang.—In numerous animals we find at the limit 
of ectoderm and mesoderm certain elements possessing the characters of nervous and 
muscular elements. ‘These neuro-muscular cells become constricted in the middle, the 
part remaining in the ectoderm becomes sensory, the part remaining deeper purely 
contractile, and the protoplasmic bridge uniting them differentiates into a rudimentary 
nerve. ‘This view is associated with the name of HEensEn, and, with certain modifica- 
tions, has been supported by Sepawick, Hep, and Granam-Kerr. Some of its 
supporters think that the cell connection is a secondary formation and is not due 
to an incomplete division, as Hensen believes. 
3. Theory of Cell Migration and the Formation of Cell-Chains.—Ba.Four, BEarD, 
Dourn, Horrmann, and others have demonstrated during the first days of embryonal 
development, especially in Selachians, the migration of neuroblasts from the nerve 
centres into the mesoderm. At the level of the lateral line of the neural tube it is 
possible to distinguish three kinds of neuro-epithelial elements. These three, primarily 
identical, are derived from the invagination of the dorsal epithelium, and later 
differentiate into cells each of which elaborates a specific substance: (a) ganglion cell— 
the neuro-chromatin granules, (b) glia cells—the glia fibrils, (c) neuro-formative cells 
or peripheral neuroblasts—the conducting fibrils. These last alone give origin to 
nerve fibres by migrating and in their further proliferation arranging themselves into 
uninterrupted cell-chains. The evolution of the nerve fibre according to this view 
shows three phases: fusiform embryonic cells, the union of these into long nucleated 
plasmodial bands, and the subdivision of these into segmented elements—the inter- 
annular segment. 
It is necessary to note, firstly, that the supporters of the first view do not deny 
the importance of the periphery in forming the path for the nerve fibres, and, secondly, 
that the supporters of the second view do not disclaim the influence of the ganglion 
cell upon the differentiation of the primary protoplasmic connections into nerve fibres. 
mi 
Ned he 
amg pero’. 6 , 
