NERVE CELLS OF THE CRAYFISH 51 
Sections of cells cut in the appropriate plane show the intra- 
cellular axone and its position in relation to the nucleus. Prep- 
arations by Bensley’s A. O. B. method show the origin of the 
axone to be very widespread throughout the cytoplasm as a 
diffusely spreading basket of neurofibrillae within which the 
nucleus lies (figs. 1 to 6, 15, 16). The composite picture pro- 
duced by a study of various sections, stained with thionin, with 
silver, and with acid fuchsin, respectively, is almost identical 
with the one seen in the acid fuchsin preparations. 
One notable difference between the origin of the axone in the 
crayfish and in most vertebrates and in some invertebrates, as 
leech and earthworm, is the difference in the locality where the 
fibrillae unite to form the compact axone. In the vertebrate, 
the fibrillae usually pass from the region about the nucleus to 
the point of exit at the periphery of the cell, where they form the 
axone. In the crayfish, on the other hand, the axone is formed as 
a tract within the cell body, receiving additional fibrillae along 
its course even to its exit. Another difference is that the perinu- 
clear reticulation in the vertebrate cell seems to be closed, that 
is, fibrillae form a closed zone about the nucleus, while in the 
crayfish most of the fibrillae of the perinuclear reticulation, 
possibly all, have a free origin within the cytoplasm. 
While it is generally known that mitochondria are found 
among the closely crowded fibrillae of the axone hillock or the 
intracellular axone whereas the Nissl substance is wanting, yet 
no explanation of the fact has been given. 
Nissl substance 
Much confusion has arisen concerning the Nissl substance 
_ because of the purely morphologic point of view of many investi- 
gators (Cowdry, 712, p. 18). This substance is the only constit- 
uent of the cytoplasm that has been placed upon a firm chenfical 
basis. Held (95, ’97) was the first to study it from this point of 
view: and to postulate its nucleoprotein character. In 1897 
Mackenzie reported the presence of organically bound iron in 
many of the Nissl bodies by the means of the Macallum micro- 
chemical test for iron. This work was greatly extended by Scott 
