PRENTISS, Weurofibrillar Structures. 161 
of infinitely smaller conducting elements, the primitive fibrillae, 
which form networks in the cells. He also figured cases of 
direct communication between the processes of nerve cells in 
the intestine of Pontobdella and instances of fibrillar networks 
in the neuropil of A/zrudo. 
In addition APATHY maintains the existence of large motor, 
and small sensory, fibrillae; these two types of fibrillae are 
connected with each other by networks in the ganglion cells 
and by the diffuse fibrillar network which, according to APATHY, 
forms the neuropil proper. 
The existence of the neurofibrillae is now generally admit- 
ted. BETHE (’98) confirmed ApAtruy’s observations as to the 
presence of the fibrillae in both the nerve cells and fibers, but 
could not distinguish between motor and sensory fibrils in the 
crab, Carcinus. He also asserts that the fibrillar networks in the 
neuropil are not diffuse. His series of studies on the neuro- 
fibrillae in the nervous elements of vertebrates (’98a, '99, : 00) 
leads him to the conclusion that they are invariably present, 
but that the networks characteristic of invertebrate nerve cells 
are rarely found. He suggests that the neurofibrillae of the 
cells may be directly connected with the fibrillar ‘‘Golginetze’’ 
which surround the cells, and that these in turn may be in com- 
munication with the fibrillae of other (sensory) elements. 
Neither BETHE nor Nissi was able to demonstrate a clear case 
of such fibrillar connection, and the assumption that the 
‘‘Golginetze’”’ are composed of neurofibrillae has been severely 
criticized by Ramon y Caja and others. NIssL (: 03) assumes 
the existence, in the central organ, of ‘‘nervous gray”’ struc- 
tures, the differentiated products of nerve cells, corresponding 
perhaps to ApATHY’s diffuse fibrillar network. The assumption 
that such nervous elements exist, is based entirely on theoreti- 
cal grounds. He points out that it has never been proved that 
the neurofibrillae of the nervous system are integral parts of 
the nerve cells, but that there are facts which indicate that they 
are not: (1) the fibrillae are sharply marked off from the rest 
of the cell in both structure and staining qualities; (2) the axis- 
cylinders are prolonged far beyond the limits of the cell proto- 
