THE CENTRAL NERVOUS SYSTEM. 335 



constituting Deiters's or spider cells. The spongioblasts im- 

 mediately around persistent parts of the neural canal retain their 

 inner connection and form a continuous layer of lining elements, 

 which later constitute the ciliated columnar epithelium of the 

 ependyma. 



The development of the nerve-fibres includes the origin of 

 two sets of primary fibres those derived from the nerve-cells 

 of the medullary tube and those growing out from the cells of 

 the ganglia. All nerve-fibres are formed as the direct exten- 

 sions and continuations of the processes of the neuroblasts. 

 In the case of those proceeding from the neural canal the fibres 

 grow peripherally and the cells remain attached to their central 

 ends, thus early establishing the relations afterwards existing between 

 the motor cells and the fibres ; those originating from the ganglia, 

 on the other hand, grow in two directions, towards the periphery 

 and towards the nervous axis, representing the sensory paths. 



The early nerve-fibres consist for some time of the axis-cylin- 

 der alone, the neurilemma and the medullary substance being not 

 only much later acquisitions but also contributions from the meso- 

 derm. The neurilemma first envelops the ectodermic axis-cylin- 

 der as a delicate sheath, and subsequently within this envelope the 

 myeline of the white matter of Schwann is deposited. The ap- 

 pearance of this coat is often very late, and takes place at different 

 times for the various tracts of nerve-fibres, although the period at 

 which the several groups acquire their medullary substance is con- 

 stant and definite for each set. The young fibres soon collect 

 into groups, which represent the early nerve-trunks, whose further 

 growth proceeds in a straight path corresponding with the general 

 direction of the component axis-cylinders ; a course once established 

 is maintained until arrested by some obstacle or modified by changes 

 in the position of the parts with which the nerve has formed attach- 

 ments. The terminations of the growing nerves are abrupt, the 

 finer ramifications appearing only after the trunk has undergone 

 repeated branchings. 



