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G, H. Parker 
to the dorsal side of the fourth ganglion, and, finally, through the 
dorsal part of the optic nerve to the brain. The series beginning 
at the anterior side of the retina (Pig. 59, black) passes from that 
side of the retina to the corresponding side of the first ganglion, 
then to the posterior side of the second ganglion, to the anterior side 
of the third ganglion, to the ventral side of the fourth ganglion, and, 
finally, through the ventral part of the optic nerve to the brain. 
The series starting from near the middle of the retina (Fig. 59, blue) 
passes through the middle regions of all four ganglia and probably 
of the optic nerve. 
e. Numerical Relation of Nerve Fibres. 
From several standpoints it vrould he of considerahle interest 
to determine the number of neurons in the series of five Orders 
extending from the retina to the brain. The arrangement of the 
nerve fibres within the ganglionic mass is, however, so complicated 
that such an enumeration was impossible and I succeeded only in 
ascertaining the number of retinal fibres and the number of fibres 
in the optic nerve, i. e., the number of fibres that enter the optic 
ganglia as a whole and the number that emerge from them. 
The determination of the number of fibres that leave the retina 
is a comparatively simple process. As previously shown each omma- 
tidium giv^s rise to seven fibres; as tbe number of ommatidia equals 
the number of corneal facets it follows that seven times the number 
of corneal facets must equal the number of fibres leaving the retina, 
These fibres remain distinct from one another in tbeir passage to 
the first ganglion, and hence as many fibres enter that ganglion as 
emerge from the retina. 
The enumeration of the fibres in the optic nerve isbyno means 
so simple an Operation. This nerve, it will be remembered, is com- 
posed of two parts (PI. 2 Fig. 38), in one of which the outlines of 
the fibres are clearly marked and their enumeration a simple matter 
of counting, while in the other the fibres are so indistinctly outlined 
that, though I have attempted to count them, I believe a method of 
estimates has yielded better results. In making these estimates the 
number of fibres and ommatidia were accurately compared in two 
crayfishes of different ages. The two specimens on which the 
enumerations were carried out may be called specimen A and spe- 
cimen B\ the former was a crayfish 3 centimetres long, and the 
latter was 7.9 centimetres in length. In both the right eye was studied. 
