428 



SPECIAL HISTOLOGY. 



Fig. 166. 



not occur to Schwann, viz., a chemical metamorphosis of the external 

 portion of the contents of the embryonic fibres ; arid the axis-fibre may 

 be only the remainder of those contents which has not undergone a fatty 

 metamorphosis. It is difficult to determine which of these two views is 

 correct. Direct observation shows only this much, that the contents of 

 the pale embryonic fibres invariably, by degrees, obtain dark contours, 

 and ultimately present the aspect of a true dark-bordered fibre, whilst 

 it teaches nothing with respect to the proper origin of the white sub- 

 stance. 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 condi- 

 tions 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 of a line, 

 which here and there anastomose, 

 all finally terminating in free 

 fibrils of the finest kind, measur- 

 ing 0-0002-0-0004 of aline. There 

 is no difficulty in showing that 

 these fibres arise from the coa- 

 lescence of fusiform or stellate 

 cells, for, in the first place, such 

 cells may be seen, in part, still in 

 close apposition with, but inde- 

 pendent of them; in part more or 

 less connected by means of their 

 processes ; and, secondly, cell-nu- 

 clei occur at the divisions of the 

 fibres, which are there somewhat 

 dilated; and, at all events, in 

 young larvae, with them are asso- 



Fm. 166. Nerves from the tail of a Tadpole, magnified 350 diameters: 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. 



