- 17 - 



have reached a higher development. We find in the ventral 

 ganglion-cord the two usual kinds of nerve-cells, lying the 

 one amidst the other. 



(1) Cells with smaller, generally oblong nuclei, mea- 

 suring '01 mm. by '007 mm., without visible protoplasm; 

 these form the chief part of the brain behind the accessory 

 brain. It is impossible to distinguish these nuclei from 

 those of the germ-layer or from the neuroglia. 



(2) Cells with larger, generally round nuclei, measuring 

 •015 mm. in diameter; these nuclei do not generally stain 

 so deeply with hematoxylin. They are usually surrounded 

 by protoplasm, and often have very distinct micro-nuclei. 

 They are usually to be found near the commissural 

 substance. They much resemble the ganglion-cells of other 

 animals. 



The real meaning of the just-described process of 

 development becomes clearer when we compare the develop- 

 ment of the central nervous system of the Arachnids with 

 that of other classes of animals. Since the Arachnid brain 

 has reached such a high degree of perfection, we must 

 look for analogies chiefly in classes of similar develop- 

 ment of brain. First we may look to the Vertebrates. 



In the central nervous system of Vertebrates and 

 Arachnids the grey and the white matter seems to have 

 totally different relative positions; the white substance 

 surrounding the grey in Vertebrates, and being almost 

 surrounded by it in Arachnids. T.,is seeming dissimilarity 

 is soon explained. Since the central nerve-tube of Verte- 

 brates 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 Mitteizoue". 

 He terms the outer of the two zones without nuclei the 

 „Randschleier u , the inner the „Saulenschicht u . These three 

 layers or „Zonen u are all to be met with in Arachnids, 



