TISSUES OF THE BODY. 343 



by the fusion of stellate formative cells (provided usually with three 

 processes), with the terminating portion of the already formed fibres, the 

 latter growing by the addition at their periphery of ne\v cells. The tail 

 of the tadpole and electric organ of the torpedo were put forward as 

 suitable objects for the recognition of these points. And, indeed, here we 

 have the best opportunity at this great distance from the central organs 

 of coming upon younger and younger specimens of nervous branches. 



In the tail of the tadpole (fig. 324) we encounter isolated nerve tubes, 

 which bear all the characters of Remaps fibres, showing nuclei situated 

 one behind the other (1). Others (2 b) without any thickened envelope 

 appear dark and medullated in the upper part, while below they become 

 finer, and are continuous with the peripheral formative cells (b l ft 2 ), 

 which radiate with their pointed processes in the. surrounding tissues. 

 Again Ave may meet, and by no means unfrequently, with nerve fibres 

 possessing thickened envelopes (2 a) and dark medulla, which is con- 

 tinued below into a progressively paling fibre (2 a 3 and a 4 ) resembling an 

 axis cylinder. 



Now, although we do not yet possess a satisfactory knowledge on these 

 points, nevertheless we have acquired enough material to demonstrate the 

 untenableness of these earlier views. 



Bidder and Kupffer, in their inquiries into the origin of the spinal 

 cord, found that the formation of nerve tubes from one row of cells occurs 

 neither in the white substance of the organ, nor in the roots of the spinal 

 nerves. In place of these fibrillse only are observed, without nuclei and 

 cells. These, the axis cylinders of the future, according to the authors in 

 question, grow simply outwards towards the periphery. The envelopes 

 appear to be formed for themselves subsequently, from new tissue elements 

 appearing between these fibrillae. 



The late excellent observer Remak also maintained, many years ago, 

 quite a different mode of origin for the ramifications of nerves in the tail 

 of the tadpole from that described in the text. According to him, the 

 branching rudiments of the cutaneous nerves appear everywhere to be 

 prolongations from the spinal ganglion. 



According to Hensen, also, the nervous ramifications in this well- 

 trodden locality are present from the commencement all the way down to 

 the periphery, in the form of fine, lustrous forked fibres (axis cylinders) with- 

 out a sign of nuclei. It is only subsequently that the mode in which they 

 become sheathed in thin, pale, and extremely elongated cells, can be recog- 

 nised, until eventually the axis cylinder lies in the interior of a nucleated 

 envelope, the stellate cells spoken of taking no part in the process. 



Besides a great instability in their contents, owing to which the latter 

 may assume the appearance of a chain of separate drops, the newly-formed 

 nerve tubes are remarkable for their great fineness as contrasted with 

 corresponding elements in the mature body. The increase in thickness of 

 the whole nervous trunk is sufficiently explained by the augmentation in 

 the diameter of the individual primitive fibres. According to Harting, 

 their thickness, in the median nerve of a foetus at four months is only 

 0-0024 rnm., while in the infant and adult they measure respectively, on 

 an average, O'OIOS and 0'0164 mm. The number of primitive tubes, at 

 these three periods, were estimated by him at 21-432, 20-906, 22-560. 



It is a well-known fact that nerves, on being severed, cease to fulfil 

 their functions, but after a certain time has elapsed regain their powers. 

 The separated ends, namely, heal rapidly ; yes, and even after a tolerably 

 23 



