466 THE AMERICAN NATURALIST. [VoL. XXXIV. 
made. First, embryology has shown that the nervous system 
in the beginning is a mass of essentially independent cells, 
some of which by throwing out processes to form nerve fibres 
and by other modifications become converted into specially dif- 
ferentiated nervous cells, or neurones. Secondly, the cellular 
character of each neurone is asserted in later life in that all 
parts of the neurone remain dependent upon their connection 
with the nucleus for continued existence and are independent 
of adjoining neurones. This is seen in the peculiarities of the 
degeneration and regeneration of nerve, operations that are 
paralleled in experiments on protozoan cells. 
Notwithstanding the obviousness of the cellular nature of 
the neurone, it must not be forgotten that these cells are of a 
very exceptional character. Imagine a cell whose nucleated 
body, scarcely visible to the naked eye, is lodged in the spinal 
cord of some such animal as an elephant, and whose process, in 
the form of an axis cylinder as fine as gossamer, stretches from 
this place through meters of flesh to the animal’s foot, and yet 
the relation of the most distant part of this process to the cell 
body is so subtile that, should the connection be anywhere sev- 
ered, the degeneration of the disconnected part invariably fol- 
lows. It is not strange that such cells as these were not 
clearly understood by the earlier histologists, for even now 
they may well excite our wonder. 
The objections to the neurone theory are naturally of very 
recent origin. One of the first to be raised was that of the 
direct union of cell 
bodies through their 
coarser processes. 
This was found by 
Dogiel to occur in the 
ganglionic layers of 
the retina (Fig. 3) 
F16. 3.— Two ganglion cells from the verte! inashowing and similar conditions 
protoplasmi (slightly modified from Dogiel). vere recorded by 
other investigators in various nervous organs; but these in- 
stances have always proved exceptional, and, while it must be 
admitted that nervous cells are sometimes united directly, such 
y DE. 
Wal 
BSN 
