Body in Vertebrates and Arthropods. 



245 



there figured shows that the cord is early dififerentiated into 

 the internal grey mass, consisting of round cells, enveloping 

 the spinal canal, while the cortical white substance or column 

 surrounds the mass of ganglion-cells. In the Annelidan 

 worms and the Arthropods the embryonic ganglion is a much 

 simpler structure, consisting of a mere mass or ball of gan- 

 glion-cells with incipient fibres passing from them. Certain 

 of these fibres iixow lono-er, forming the commissures, trans 

 verse and longitudinal, connecting the ganglia. At first, 

 then, the nervous system of the higher worms (those with a 

 ganglionated chain) and Arthropods consists of a series of dis- 

 connected ganglia, which eventually become connected by 

 secondary products, the commissural fibres. The fact that in 

 Worms the brain is at first separated from the rest of the 

 ganglia, as stated in Balfour's 'Embryology' (i. p. 291), is 

 not of particular significance, since all the ganglia, at least 

 in Crustacea and insects, are at first disconnected from each 

 other. 



Embryology appears to give no countenance to the view 

 held by some authors that the brain of an Arthropod may 

 represent the nervous system of the Vertebrate, and the post- 

 cesophageal chain of ganglia the sympathetic system of the 

 Vertebrates. 



Tiiere seems to be a unity of plan, so to speak, in the 

 development of the nervous system of the Arthropods, and how 



Fis-. 1. 



Early stage of Ascidian embryo, showinp: the nervous tube n, open in 

 i'ront and situated dorsally above the alimentary tube {h), as in 

 Vertebrates, 



radically difi'erent that is from the mode of genesis of tlie 

 vertebrate nervous system may be seen by reference to 

 Balfour's work (ii, pp. 250-252) or those of other observers. 

 "While the nervous system of all animals arises from the ecto- 

 derm (epiblast), as Balfour states : " In all Chordata an axial 

 strip of the dorsal epiblast, extending from the lip of the 



