DEVELOPMENT OF FIBRES. 387 



stead of being striped (all the muscles of insects are of the 

 striped kind), are partly striped, partly unstriped ; that is 

 to say, in the same bundle some of the fibrillse are without 

 the transverse markings, and those fibrillge which have 

 such markings have them only part of the way down, the 

 remainder of the fibrillse being imstriped. This is not only 

 interesting as a fact in muscle development, but presents 

 a striking analogy to the development of nerve-fibres, which 

 we here see in the same trunk partly emerged from their 

 primitive granular condition. 



I conclude, therefore, that the differentiation of nerves 

 shows the following phases : 1st, As in many Molluscs and all 

 embryos, a granular homogeneous mass ; 2d, As in Insects, 

 and perhaps Crustaceans, a linear disposition of the granules 

 into fibres, but without an investing sheath ; 3d, Fibres, or 

 rather tubules, differmg from the preceding in structure, hav- 

 ing each an enveloping sheath, which isolates one fibre from 

 the other, so that the nerve becomes a fasciculus of tubules* 



It would lead us too far to follow the many applications of 

 these facts to the vexed questions of nervous histology and 

 physiology. The hotly-debated controversy respecting the origin 

 of fibres as prolongations from ganglionic cells, for example, 

 seems to me decisively settled by the fact that, in the Mol- 



* Stilling, op. cit. p. 11-13, decides that all primitive fibres have an invest- 

 ing sheath ; but unless he would deny the claim of those named in the second 

 class to be considered as fibres, he is certainly wrong. It should be noted that 

 what are called fibres in the vertebrate ncrvc-tissue are really tubules with fluid 

 contents. This fact was discovered by Leeuwenhoek, who describes them as 

 vessels (see his Select Works, ii. 303), a discovery often erroneously attributed 

 to Ehrenberg, to whom the credit is given in Mr Hogg's popular work, The 

 Microscope, 3d edition, p. 525. 



