No. I.] EYES OF ARTHROPODS. 217 



nerve-fibres, would correspond to the primitive ocellus with its 

 optic nerve, while the dorsal half of the compound eye would 

 correspond to the dorsal extension of the ocellus. 



Just as in the early stages oi Acilius, whose ocelli we shall re- 

 gard as typical, the ventral half of the double ocellus, or the 

 primitive ocellus, is alone supplied with a special nerve, so in 

 the beginning, the ventral half of the compound eye in Vespa is 

 likewise the only part supplied with a special bundle of nerves. 

 The comparison may be carried farther, for in the next stages 

 of both organs, each half has a special nerve-branch. Still later, 

 the two parts of the ocellus become practically independent, 

 being separated by a layer of indifferent cells. The same oc- 

 curs in the compound eye, which, as I have shown, is divided 

 into a dorsal and ventral half by a median cord of indifferent 

 cells. Again, the dorsal half of the ocellus, being a mere exten- 

 sion of the original ocellus, must be regarded as a later formation. 

 The correspondence of the dorsal half of the ocellus with 

 the similarly situated part of the compound eye is shown by 

 the fact that the dorsal half of the latter is the last to develop 

 ommatidia. Still another point of resemblance between the 

 compound eye of Vespa and the double ocellus of Acilius is to 

 be seen in the development of the dorsal and ventral folds which 

 enclose the eye. We have shown that the ocellus in Hydrophi- 

 lus arises as a pouch of epidermal cells pushed ventrally 

 beneath the epidermis. No fold is formed on the dorsal side 

 of the ocellus (Fig. 10, ah). Its closure is effected by the growth 

 of a ventral fold dorsalwards until it unites with the dorsal edge 

 of the optic cup. But it seems that the complete closure of 

 the optic cup does not take place, if at all, until some time 

 after hatching. Meantime, however, the dorsal edge of the 

 retina has travelled dorsally (Fig. 10, ab), but is finally over- 

 taken by the ventral fold (Fig. 10, z), which is growing in the 

 same direction, and perhaps completely fuses with it, although 

 it is possible that a complete fusion never takes place, the sub- 

 sequent extension of the ocellus being effected by the dorsal 

 growth of the ventral fold and by the dorsal edge of the retina. 

 The inclosing and growth of the compound eye is effected 

 in a similar manner. The development of the eye is initiated 

 by the formation, in a thickened patch of ectoderm, of a ventrally 

 directed pouch. The pouch is consequently covered on its 



