The Eetina and Optic Ganglia in Decapods, especìally in Astacus. 57 
degree of complication could be arrived at. It is the purpose of 
this section to attempt such a formulation. 
The growth of the optic tracts in Astacus can be satisfactorily 
studied only at two places, the retina and the optic nerve. These 
will be considered separately, beginning with the growth of the retina. 
The retina in an adult crayfish, as will be remembered, has an 
elongated form, its anteroposterior dimension when measured in om- 
matidia being about 58, its dorsoventral extent, when similarly mea- 
sured, approximating 31. If, with the intention of ascertaining how 
these dimensions bave been produced, the corneal cuticula on the 
margin of the retina be examined, it will be seen that on the dorsal, 
ventral, and posterior edges the transition from the faceted to the 
non-faceted cuticula takes place abruptly or at least with the inter- 
vention of only a very few imperfect facets (PI. 1 Figs. 30, 31, 32) ; 
on the anterior edge (Fig. 33), however, the region of transition is 
conspicuous and contains a considerable number of irregulär facets 
and outlines of cells apparently in process of forming facets. This 
condition is characteristic of the whole anterior edge as contrasted 
with the other three edges, and suggests at once that the excessive 
anteroposterior extension of the retina is due to continued growth on 
this edge. 
To ascertain whether the retina has grown in this way or not, 
the eyes in several young crayfìshes were examined. In a newly 
hatched specimen measuring 8.5 mm. from the tip of its rostrum to 
the end of its telson, the retina contained in its dorsoventral curva- 
ture 29 ommatidia and in its anteroposterior 27. When these num- 
bers are compared with the corresponding ones in an adult, it will 
be observed that, though at this early stage the retina has acquired 
almost the full number of ommatidia in its dorsoventral curvature, 
in its anteroposterior dimension it has only about half as many as 
it must bave ultimately; so that, between this stage and that of an 
adult, the ommatidia must approximately double their number and 
the growth must be either on the anterior or on the posterior edge 
of the retina or on both. When these two edges were compared in 
the young crayfish just mentioned, the mass of hypodermal cells, 
as well as the occurrence of small ommatidia on the anterior edge 
and the absence of these peculiarities from the posterior one, left 
no doubt that only the anterior was concerned in the extension of 
the retinal area. 
In an older crayfish whose length was 2.4 cm., the dorsoventral 
