No. I.] EYES OF ARTHROPODS. 1 95 



ends, as though forcibly pinched in by the advancing ec- 

 toderm. 



The cephahc lobes, before they are completely shut off from 

 the exterior, are composed of distinct groups of spindle-shaped 

 cells, which strongly resemble certain kinds of sense-organs. 

 These groups of cells, whatever their significance may be, divide 

 rapidly, and subsequently form quite regularly arranged balls 

 of cells, which give the brain, when cut in certain directions, a 

 segmented appearance somewhat like that of the ventral cord. 

 The number and fate of these nerve-balls I have not been 

 able to determine with certainty. There are at least six for 

 each lobe ; three of them undoubtedly form the optic ganglia ; 

 it seems probable that the fourth develops into the antennary 

 lobe, and the remaining two into the mushroom-shaped bodies 

 on the dorsal side of the brain. The thin layer of ectoderm 

 continues to advance over the cephalic lobe until it has en- 

 closed all but an oblong space, whose long axis is parallel with 

 that of the lobe, on the median part of its ventral edge. This 

 still uncovered part is deeply depressed, and is composed of 

 three cords of cells (Fig. 2, op.g.^~^). The middle cord 

 is widest at the proximal end of the lobe, and gradually 

 narrows toward the opposite extremity, where the other two 

 cords are widest. I shall speak f 1 these three cords which 

 ultimately develop into the optic ^anglion, as the injier, middle 

 and outer walls of the ganglionic fold. 



The superficial ectoderm finally bridges the ganglionic fold 

 and coalesces with the undifferentiated ectoderm on the ventral 

 edges of the cephalic lobes. It is worthy of remark that in- 

 stead of the brain being formed in the usual way from the inner 

 of two layers arising from the splitting of the cephalic lobes, 

 the cephalic lobes themselves are depressed and finally covered 

 by an advancing sheet of ectoderm, the edge of which was 

 formerly continuous with the dorsal and distal edges of the 

 cephalic lobes (Figs. 1-3). 



The ectoderm, at the point where it is continuous with the 

 ventral edge of the cephalic lobes, and therefore in close prox- 

 imity to the ganglionic fold, becomes slightly thickened to 

 form the foundation of the compound eye (Fig. i, E). 



Before the advancing sheet of ectoderm has covered the gan- 

 glionic fold the latter is continuous with the thickened patch of 



