THE NERVOUS SYSTEM. 



is;» 



teaches nothing with respect to the proper origin of the white 

 substance. Since, however, it can be proved, that the fibres, 

 whilst they undergo this change, do not alter in size, the 

 supposition I have expressed would still appear the more 

 correct. 



The development of the terminations of the nerves, which 

 appears in some respects to 

 present conditions different 

 from those exhibited in the 

 trunks, may, as I have 

 shown (' Annal. d. sc. nat./ 

 1846, p. 102, tab. 6, 7), be 

 readily traced in the tails 

 of the larvse of the naked 

 Amphibia (fig. 165, 3 ; fig. 

 166). We there find, as is 

 mentioned by Schwann (p. 

 177), the primary rudiments 

 of the nerves to be pale 

 branched fibres, measuring 

 0-001— 0-002'", which here 

 and there anastomose, all 

 finally terminating in free 

 fibrils of the finest kind, mea- 

 suring 0-0002 — 0-0004.'". 

 There is no difficulty in 

 showing that these fibres 

 arise from the coalescence 

 of fusiform or stellate cells, 

 for, in the first place, such 

 cells may be seen, in part 

 still in close apposition with, 

 but independent of them; in 

 part more or less connected by means of their processes ; and, 

 secondly, cell-nuclei occur at the divisions of the fibres, which 

 are there somewhat dilated; and, at all events in young larvee, 



Fig. 166. Nerves from the tail of a Tadpole, x 350 diam.: 1, embryonic nerve- 

 fibres, in which more than one dark-bordered tube has become developed; 2, similar 

 fibres containing but one tube, which in one fibre ceases at b ; 3, embryonic pale 

 fibres; 4, fusiform cells connected together, and with a complete nerve-fibre. 



