72 
G. H. Parker 
Fig. 51. Termination of a fibre from the optic nerve in its ganglionic cell and 
in fibrillations in the fourth optic ganglion. Methylen blue. X 140. 
Fig. 52. Termination of two optic nerve fibres in ganglionic cells of the fourth 
ganglion without apparent fibrillations. Methylen blue. X 140. 
Fig. 53. Termination of an optic nerve fibre in a fibrillation in the fourth ganglion 
without any apparent connection with a ganglionic cell. Methylen blue. 
X140. 
Fig. 54. Fibrillation of an optic nerve fibre in the optic lobe of the brain. Eapid 
Golgi method. X 140. 
Fig. 55. Transverse section of the fourth optic ganglion showing the dorsal 
eminence {x) into which pass fibres from the dorsal part of the optic 
nerve {n.d) and from the cells [cl.gn] on the anterior face of the ganglion. 
Vom Eath's method. X 
Fig. 56. Longitudinal section of the fourth optic ganglion showing the dorsal 
eminence {x) and the dorsal part of the optic nerve {n.d). Vom Rath's 
method. X 
Fig. 57. An outline of portioüs of a longitudinal section of optic ganglia in 
which numerous fibres had been stained with methylen blue: fixed in. 
corrosive sublimat; x a nerve fibre with an anomalous course. X 
Fig. 58. Nervi nervorum (?) from the surface of the fourth ganglion. Methylen 
blue. X 140. 
Plate 3. 
Fig. 59. A diagram representing the nervous structure of the optic tracts in 
Astacus. The optic stalk is supposed to be cut on its axis in an antero- 
posterior plane; in the fourth ganglion and in the optic nerve those 
fibres and cells that would lie dorsal to the plane of section are drawn 
in solid colors; those ventral to this plane are represented in dotted 
lines. Whereas the path indicated by the green fibres involves only 
three neurons between the retina and the brain, the other paths require 
five. Although the diagram represents the actual conditions rather fully, 
it must be born in mind (cf. pag. 52) that for each nerve fibre entering the 
brain, there are about four emerging from the retina. The arrows 
indicate the direction of growth. X 40. 
Fig. 60. A diagrammatic figure of a neuron of the first order (cf. pag. 20); the 
nervous portions are colored red. 
Fig. 61 — 66. Diagrams to illustrate the transition from a simple (Fig. 61) to a 
complex ommatidium (Fig. 64). Dififerent kinds of cells are designated 
by difi'erent colors: cone cells, green; the dorsal undifferentiated reti- 
nular celi and its derivatives, the distai retinular cells, blue; the four 
remaining unditferentiated retinular cells and their derivatives, the 
proximal retinular cells, red; other parts, gray. The longitudinal 
sections (Figs. 61 and 64) are supposed to be cut in the dorsoventral 
piane. The transverse sections are placed with their dorsal sides upper- 
most. The numbering of the retinular cells is such that in the simpler 
type each cell that gives rise by division to two cells, is indicated by 
the same numbers as are used to designate its descendants. 
Fig. 61. Longitudinal section of an ommatidium of the simpler type. 
