150 PATTEN. [Vot. II. 
of a formerly sensory cell lodged in the ectoderm, or of one derived 
by division from such acell. In passing through the medulla this 
prolongation breaks up into a bundle of fibrille. The other two 
prolongations are small, and contain no granular protoplasm ; 
they arise from the opposite sides of the broad end of the pear- 
shaped cells and, extending tn opposite directions, probably serve to 
unite the ganglion-cells with one another. 
COMPARATIVE ANATOMY OF THE OpTiIc GANGLION. 
Although some work has recently been done on the develop- 
ment of the Arthropod eyes, the optic ganglion, both of the con- 
vex eyes and ocelli, has received little notice. It has been stated 
that it arises either as an invagination of the ectoderm near the 
eyes, or as an outgrowth of the brain. Both Bobretzsky and 
Reichenbach have failed to produce satisfactory evidence to 
show that what they regarded as the developing optic ganglion 
really was such. Reichenbach made the fatal mistake of neglect- 
ing to follow his optic invagination up to a point where its real 
nature would be apparent. Had he done so, he would not have 
mistaken the retinal ganglion for the layer of rhabdoms and re- 
tinulz, and he would have seen that his crystalline-cone layer was 
the whole ommateum. In my paper upon the eyes of Vespa, I 
maintained that Reichenbach had misinterpreted the facts. I 
have recently examined, by means of sections and ‘surface views, 
the head of Astacus, and find my objections sustained. 
It is not my intention to enter here into a long account of the 
comparative anatomy of the optic ganglion of Arthropods, for 
the subject is worthy of separate consideration. I merely de- 
sire to state the main conclusions to which I have been led by a 
study of the development of the optic ganglion in such groups 
of Arthropods as the Isopods, Decapods, Hymenoptera, Lepidop- 
tera, and Coleoptera. 
In Vespa, it was shown that on the dorsal edge of the optic 
thickening a great mass of cells was pushed inward to form the 
rudiment of the optic ganglion. This mass of cells, which from 
the very earliest stages is connected with the ventral edge of the 
optic thickening, after invagination divides into three lobes, which 
immediately arrange themselves in a line connecting the eye and 
brain. The middle lobe develops into the optic ganglion of the 
