ON THE ORIGIN OF VERTEBRATES FROM ARACHNIDS. 335 



of the optic plate and ganglia over the brain. This fact is of 

 great theoretic importance^ especially when compared with 



Fig. 5. — A. Plan of cephalic lobes of Scorpion at an early period ; the 

 arrows indicate the directions along which the lateral lip of the ganglionic 

 pits advance. B. Semi-diagrammatic view of fore-brain of Scorpion in 

 Stage F. The left side shows the position of the deeper portions ; the right 

 side, the superficial ones. The lines a, b, c, and d represent successive 

 positions of the rim to the ganglionic pits. The shaded portion in front 

 of c represents the area of the optico-ganglionic pockets about to unite ; 

 the dotted line back of d, the sac after the union has taken place ; the 

 lateral eye-plate, /. e., represents the only part of the original cephalic lobes 

 now on the surface. A. Section through cephalic lobes of Acilius, or 

 through third brain-segment of Scorpio ; compare A, a. B. Section 

 through second segment of Scorpio, in direction b, A. C. Same at later 

 period. D. Ditto, still later, a, b, and d. Direction of sections a, b, and 

 d. b^-^. Lobes of brain. Br. Brain, c.p. Capsule of eye. ct. Cuticula. 

 g. v^-^. Ganglionic pits. g. v^., a, b. Succes- 

 l. e. Lateral eyes. m. Mouth, me. Median 

 n. I. e. Nerve to lateral eyes. n.p. Neuropore. 

 op. n. Optic nerve, o. pP-^. Optic plates. 



e. Eye. ec. Ectoderm, 

 sive positions of third pit. 

 eyes. n.f. Nerve-fibres. 



op. g^~^. Optic ganglia. 



r. Primitive retina. ?•'. Secondary permanent retina. 



what takes place in the ventral cord, where only a narrow 

 median furrow, the only part lined by primitive ectoderm, is 

 invaginated (see Fig. 9 and Fig. 2, c e). 



