152 PATTEN. [Vot. II. 
layer of clear cells, Reichenbach’s “ Augenfalte.” The shape, 
position, and structure of this fold, and the manner in which it 
degenerates and disappears, leave little room to doubt that it is 
homologous with the outer lobe of the optic ganglion of Vespa. 
The fact that in Astacus this layer is folded as if formed by 
invagination is an incidental and secondary condition. In Vespa, 
Cecropia, and Acilius the direction of the infolding is reversed. 
The middle lobe is relatively smaller in Astacus than in 
Vespa; from it arises the epiopticon of Hickson, or the second 
optic ganglion of Carriere. This lobe contains an oval medulla, 
from the outer end of which arise decussating fibres that extend 
into the retinal ganglion, The inner lobe of Astacus, unlike that 
of Vespa, develops into a distinct ganglion, with a medulla and 
cortical layer of ganglion-cells like those in the median lobe, but 
darker colored. It is over-arched by a thick layer of cells aris- 
ing from the cortical layer of the stalk of the optic ganglion. 
The distal end of the stalk is enlarged, and apparently forms 
a third medulla, the minute structure of which is quite different 
from that of the other two. 
On the outer side of the first lobe is a layer of small, dark 
ganglion-cells, which Reichenbach regarded as the outer wall of 
his “Augenfalte,’ and which he maintained gave rise to the layer 
of retinula and rhabdoms. But this layer of cells is really the 
retinal ganglion, and is formed, as in Vespa, by the development, 
some time after the ganglionic invagination, of a layer of cells 
among the fibres connecting the optic ganglion with the eye. 
Among /sopods I have examined the optic ganglion of Cymo- 
thoa, and find that in development and structure it does not 
differ from that of Astacus in any particulars that concern us at 
present. 
No extended observations were made on the optic ganglion of 
the larvee of Cecropia. I can only say that it has a superficial 
resemblance to that of Acilius, but it is much smaller and less 
perfectly developed. Toward the beginning of pupal life, the 
optic ganglion increases enormously in size and assumes a 
structure very much like that of Vespa (Fig. 2, C.). The resem- 
blance between the optie ganglion of Vespa and Cecropia is suf- 
ficiently evident to render a detailed description unnecessary. In 
the later stages both the inner and outer lobes disappear or are 
reduced to mere insignificant appendages, and the cells at the 
