40 
G. H. Parker 
b. Topography of Ganglia. 
First Optic Ganglion. The first optic ganglion (Fig. 27, I) 
is a relatively thin, dome-shaped structure with a convex distal face 
and a concave proximal one, in both of which the ciirvature con- 
forms very closely to tbat of the retina. In passing through the gang- 
lion from its proximal to its distal side, four layers can he distin- 
guished. which may be designated as follows: the external nuclear 
layer (PI. 2 Fig. 46, 2), the fibrous layer (3), the internal nuclear layer 
(4), and the layer of »Punktsuhstanza (5). The retinal fibres make 
their way into the ganglion through its distal face , and from its proxi- 
mal one emerge the fibres that connect it with the second ganglion. 
The distal nuclear layer, which forms, of course, the distal 
surface of the ganglion, consists of a number of large oval or roun- 
dish nuclei imbedded in what is apparently an irregularly fibrous 
mass (PL 2 Fig. 41). Berger (78, pag. 199) noticed in Squilla the 
resemblance between these nuclei and the nuclei of connective tissue 
cells, and much the same interpretation of them is implied by Gre- 
NACHER (79, pag. 120) in bis account of the ganglion in Mysis. 
ViALLANES (92a, pag. 395), however, who has studied them in Pali- 
nurus^ regards them as true ganglionic nuclei. In Golgi preparations 
(PI. 2 Fig. 46, fhr.r]^ the retinal fibres can be seen passing between 
these nuclei without entering into any definite relations with them. 
In preparations stained in methylen blue, the cells to which these 
nuclei belong are often clearly outlined; they are usually spindle- 
shaped with their longer axes parallel to the retinal nerve fibres, 
with which, however, they show no Connections. The fact tliat 
these cells do not send processes into the Punktsubstanz « , as 
well as their lack of Connection with nerve fibres, justifies the con- 
clusion, I believe, that they are not nervous, but rather sustentative 
in function. The layer that they form can be traced directly into 
a mass of ectodermic tissue that lies on the anterior face of the gang- 
lion, and the resemblance between the nuclei in the distal nuclear 
layer and those in the ectodermic mass is so striking that, though 
these cells are most probably sustentative in function, I believe them 
ectodermic in origin. If this is so, it is probable that we have to 
do in this instance with a tissue not unlike the ectodermic neuroglia 
in vertebrates. 
The second or fibrous layer (PI. 2 Fig. 46, 3) is composed of 
a dose feit of fibrous material almost entirely devoid of cellular 
