378 ADAM SEDGWICK. 



presents a patch of white matter at the point of entrance of 

 the optic nerve, and a layer of white matter (the rods) next the 

 lumen of the vesicle. The pigment appears in January at the 

 junction of the layer of rudimentary rods and the nuclei. The 

 lens is formed at about the same time, as a secretion of the 

 wall of the vesicle. It lies within and fills up the cavity of 

 the vesicle. This condition of the eye is practically that of 

 the adult. 



The Ventral Cords. 



The early history of the ventral cords is given on p. 475 of 

 Part III. In Stage f they are still in close contact with the 

 ectoderm, but an indistinct line of separation can generally be 

 seen between them (Part III, fig. 39). In Stage g the separa- 

 tion is complete and distinct, though they still remain con- 

 nected at intervals by cords of cells with the ventral organs 

 (PI. XXVI, fig. 5). It is in Stage g also that the commissures 

 between the ventral cords and the main nerves first become 

 apparent. The commissures between the two nerve-cords are 

 very numerous. They extend in this stage from the ventro- 

 median corner of the cords towards the ectoderm, where they 

 lie in close connection with some rather loose fibrous tissue, 

 which is found at this stage everywhere beneath the ectoderm. 

 They consist of fibrous matter, and can be easily traced into 

 the white matter of the cords. 



The efferent nerves arise from the outer border of the cords, 

 directly from the white matter. They are very numerous 

 (Balfour, No. 2), but there are, opposite each leg, two — the 

 pedal nerves — which are much larger than the others and more 

 easily observed. These two arise, the one immediately in front 

 of the nephridium, and the other behind it. In Stage g, when 

 they are first apparent, they consist of close bundles of fibres 

 passing out from the white matter (PI. XXVI, fig. 5, nerve) 

 and continuous with a loose plexus of fibres placed imme- 

 diately within the ectoderm of the ventral side of the legs 

 (neuro-musc). It therefore appears that the commissure be- 

 tween the nerve-cords, the efferent nerves, and the fibrous 



