24 OLIGOCHAETA 



of the body there ai-e three tubes ; these consist of a central nerve-fibre enclosed in 

 a double sheath. The outer sheath has a few scattered nuclei in it and has a fibrous 

 texture ; the inner sheath has the same texture but fewer nuclei. The central fibre 

 or rather bundle of fibres is the direct prolongation of certain large nerve-cells. The 

 rest of the nerve-cord is made up of fibres and cells; the latter are ventral and 

 lateral in position. In the brain of course the conditions are reversed. The fibrous 

 part of the nerve-cord consists of a more or less transparent ' cytoplasm,' the remains 

 of a poi-tion of the embryonic cell-mass ; this in places forms transverse and longi- 

 tudinal canals dividing up the meshwork of nerve-fibres into different regions. The 

 fibrous mass is surrounded and. the canals of the cytoplasm also, by a delicate sheath 

 the ' Glia sheath ' ; in this are a few nuclei ; it appears to be of the nature of 

 connective tissue and is not connected with the nerve-fibres. The ganglion cells are 

 grouped into a medial and two lateral masses; the cells in Ekynchelmis are for the 

 most part unipolar; only seldom are multipolar cells met with; in Lumbricus 

 Cerfontaine figures multipolar and unipolar cells also. 



The development of the nervous system has been studied by Kleinenbekg, 

 KovALEVSKY, WiLSON, Bergh, Ve.tdovskt, etc. Wilson discovered, and the subse- 

 quent observers confirmed him, that the cells which form the nervous system, like 

 those which will form the nephridia, originate from a single cell on each side, placed 

 near the posterior end of the body and termed a ' teloblast ' ; continuous with this 

 teloblast and forming a row of cells produced out of it is the layer which will ultimately 

 become the nervous system ; the teloblast is an epiblastic cell and there is therefore 

 no doubt as to the epiblastic character of the central nervous system in the Oligochaeta. 

 It appears to be entirely formed by the proliferation of these cells, and Wilson declares 

 that the cerebral ganglia are formed continuously with the ventral chain ; it follows 

 from the mode of origin just referred to that the ventral cord and the cerebral ganglia 

 are a double formation, that the nervous system is bilaterally symmetrical ; it has been 

 held that it is a single formation laid down in one band. Bergh, while confirming 

 Wilson, made the interesting addition to his facts that in the embryo there is a series 

 of branched cells evidently of a nervous nature, which lie between the two nervous rows ; 

 Bergh thought, and Vejdovsky confirmed him, that this plexus of cells and fibres is to 

 be traced to the ventral epiblast and has no relation to the neuroblasts already referred 

 to. Whether these cells have any relation to the definitive nervous system seems to be 

 at present a matter of some doubt. Vejdovsky looks upon this primitive plexus as 

 a remnant of the nerve-ring of the Medusa. The histological difierentiation is so 

 special a matter that I do not enter into it here. For details the reader is referred to 

 Vejdovsky's work (9). 



