J ^5 PATTEN. . [Vol. I 



ectoderm which gives rise to the compound eye (Figs, i and 2, op. 

 ^'). The ganglionic fold finally breaks away from the optic 

 thickening, the free edge of which immediately unites with the 

 advancing sheet of ectoderm to form a continuous layer over 

 the brain (Figs, 2 and 3). 



The optic thickening, therefore, does not lie in the cephalic 

 lobes, properly speaking, or those parts which give rise to the 

 brain, but just on one side of them. 



While the ganglionic fold is still uncovered, the optic nerve ap- 

 pears as a cord of cells arising from the optic thickening and 

 uniting it with the middle wall of the ganglionic fold (Figs. 1-3, 

 op.n.^. 



The inclosing of the ganglionic fold, the formation of the 

 optic nerve, and the slight depression in the optic thickening, 

 take place before the embryo hatches. 



In larvae about 3 mm. long, the optic cavity (Fig. 4. E,^ has 

 assumed a very characteristic shape, in that the invagination is 

 deepest on its ventral edge, and gradually grows shallower to- 

 wards the opposite side. The optic nerve is attached to the 

 ventral and deepest part of the invagination (Fig. 4, op.Ji.). 



In the 5 mm. larvae, the optic invagination is very deep and 

 may be recognized as such without difficulty (Fig. 5, E). The 

 optic nerve is still attached only to the deeper part of the in- 

 vagination. The ganglionic fold is, in this stage, completely 

 filled by the middle wall {op.g.^), which has pushed the outer 

 wall {op.g.^), now transformed into a layer of columnar cells, just 

 below the floor of the optic cup. 



The brain-sheath {br.sh.) forms a thin but distinct layer of 

 cells between them. 



The cells of the middle wall multiply rapidly in a remarkable 

 manner. One of the cells divides into two pear-shaped ones, 

 united at the base by slender stalks, but free at the nucleolated 

 apices. One of these cells divides again in the same manner. 

 This process is repeated many times, until a long and nearly 

 straight fibre is produced with pear-shaped cells arranged upon 

 it like leaves upon a growing shoot. These clusters of pear- 

 shaped cells, of which the whole optic ganglion is composed, 

 are unipolar ganglion-cells. The main stalks to which the 

 cells are attached, are probably composed of a bundle of fibril- 

 lae, each fibrilla being the prolongation of the stalk of a single 



