612 L. H. GOUGH. 



analogies chiefly in classes of similar development of brain. 

 First we may look to the Vertebrates. 



In the central nervous system of Vertebrates and Arach- 

 nids the grey and the white matter seem to have totally 

 different relative positions; the white substance surrounding 

 the grey in Vertebrates, and being almost surrounded by it 

 in Arachnids. This seeming dissimilarity is soon explained. 

 Since the central nerve-tube of Vertebrates becomes folded 

 inwards during its development, it is clear that its innermost 

 walls, which abut on the canalis centralis, are that part of 

 the wall which was originally outermost. Thus we would in 

 both cases have the grey substance outermost, the white 

 substance lying under it. 



His (16) describes the spinal cord of Vertebrates as arising 

 out of " Zwei Kernfrei Zonen, eine ausserste und eine innerste, 

 und eine die kerne enthaltende Mittelzone." He terms the 

 outer of the two zones without nuclei the " Randschleier," 

 the inner the '' Saulenschicht." These three layers or 

 ''Zonen " are all to be met with in Arachnids, though some- 

 what modified, having in part become discontinuous. The 

 columnal layer, " Saulenschicht," could be compared to the 

 " surface pits ; " but it, of course, no longer forms a con- 

 tinuous layer in Arachnids as it does in Vertebrates. The 

 " Mittelzone " can be compared, as regards position and struc- 

 ture, to the nuclei groups in the developing ventral ganglion- 

 cord of Admetus, and the " Randschleier,'^ which gives rise 

 to the white matter in V^ertebrates, is evidently equivalent to 

 the commissural substance, or "Punktsubstanz" of the brain 

 of Admetus, also as regards position and structure. 



The ganglion cells of Admetus are almost always to be 

 found in the depth of the grey matter, just below the Punkt- 

 substanz or white matter, as is also the rule with Vertebrates. 

 In the latter all nerves proceed out of the white substance. 

 Even this has an analogy in Arachnids, as all the nerves I 

 met with in my cuts left the ventral ganglion-cord just 

 above the grey matter, at places where the nucleiferous 

 covering of the "Punktsubstanz " was very thin. These 



