468 THE ORIGIN OF VERTEBRATES 
the secondary neurones show by their degenerative changes the 
importance of their connection with the peripheral organs. 
In this way I can conceive the formation of a series of both 
efferent and afferent relays in the nervous system by proliferation of 
the original neural moiety of the neuro-epithelial elements, every 
one of which is dependent upon its connection with the peripheral 
epithelial elements for its due vitality, the whole system being a 
scheme for co-ordination of a larger and larger number of peripheral 
elements. Thus the cells of the vasomotor centre are in connection 
with the whole system of segmental vaso-constrictor centres in the 
lateral horns of the thoracic region of the cord, so that to cause 
atrophy of these cells a very extensive removal of the vascular 
system would be required. Each of the segmental centres in the 
cord supplies a number of sympathetic segments, the connection 
with all of which would have to be cut in order to ensure complete 
removal of the connection of each of its cells with the periphery, and 
finally each of the cells in the sympathetic ganglia supplies a number 
of peripheral elements, all of which must be removed to ensure com- 
plete severance. 
Thus, if we take any arbitrary number, such as 4, to represent 
the number of peripheral organ-elements with which each terminal 
neurone is connected, and suppose that each neurone has proliferated 
into sets of 4, then a cell of the third order, such as a cell of the 
vasomotor centre, would require the removal of 64 peripheral elements 
to cause its complete separation from the periphery, one of the 
second order (a cell of the thoracic lateral horn) 16 elements, one of 
the first order (a cell of a sympathetic ganglion) 4 elements. 
Such intimate inter-relationship between the neurones, both 
afferent and efferent, and their corresponding peripheral organs does 
not imply that all nerve-cells are necessarily as closely dependent 
upon some connection with the periphery, for just as the proliferation 
of epithelial or muscle-cells forms an epithelial or muscular sheet, 
the elements of which are so loosely, if at all, connected together that 
their metabolism is In no way dependent upon such connection, so 
also a similar proliferation of the neural elements may form con- 
nections between nerve-cell and nerve-cell of a similarly loose 
nature. 
It is this kind of proliferation which, in my opinion, would bind 
together the separate relays of efferent and afferent neurones, and 
