130 CHANGES FROM STAGE G TO BIRTH. 



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. 57. 

 In Stage f they are still in close contact with the ecto- 

 derm, but an indistinct line of separation can generally be 

 seen between them (PI. VIII, 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 

 (PL X, 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, 

 dii'ectly from the white matter. They are very numerous 

 (Balfour, No. 20), 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. X, 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 commissures be- 

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

 matter beneath the ectoderm, all become distinctly apparent 

 at about the same time in Stage g ; but how and when they 

 are developed I am unable to say. As may be seen from an 

 inspection of the sections (PI. VIII, figs. 37 — 39) there is in 

 Stage F a certain amount of this fibrous tissue, especially at 



