CHAP. Vlll.] DEVELOPMENT OF NERVOUS MATTER. 



227 



fibres, we can offer no remark. In insects and myriapoda, the nerve-tubes 

 vary considerably in size ; they are collected into bundles, and are surrounded 

 by a transparent sheath of homogeneous membrane, which accompanies the 

 larger ramifications of the nerve-trunks. The white substance of Schwann is 

 not so obvious nor so constant in these nerves as in those of the lobster, and 

 the existence of nuclei (fig. 64) makes them resemble closely the gelatinous 

 fibres of the vertebrata. The anatomical characters of the vesicular nervous 

 matter of invertebrata do not essentially differ from those of the same sub- 

 stance in the vertebrate classes, so far as our observation enables us to judge. 

 The nerve-vesicles with nuclei and nucleoli are equally apparent in both, 

 though in the former they are more transparent, and contain less pigment. 



Of the Development of Nerve-fibres. We can add nothing to 

 the account given by Schwann of the development of nerve. The 

 following is quoted from Dr. Willis's translation of Wagner's Phy- 

 siology : 



" The nerves appear to be formed after the same manner as the 

 muscles, viz. by the fusion of a number of primary cells arranged 

 in rows into a secondary cell. The primary nervous cell, however, 

 has not yet been seen with *& G5 - 



perfect precision, by reason 

 of the difficulty of distin- 

 guishing nervous cells whilst a 

 yet in their primary state, 

 from the indifferent cells out 

 of which entire organs are 

 evolved. When first a nerve 



f? "*fcjk 



can be distinguished as such, 

 it presents itself as a pale 



* . Various stages of the development of nerve : .. ^cm.^oi 



COrd With a longitudinal stage, ft. Detached fibre, c. Nucleated fibre in the lower 



. . . n part of which, d, the white substance of Schwann has begun 



fibrillation, and 111 thlS COrd to be deported, e. Nucleus in a more fully-formed fibre 



.. . .. _ , . between the white substance and tubular membrane. 



a multitude OI nuclei are / Displays the tubular membrane, the contained matter 



. ._. . having given way. (After Schwann.) 



apparent (fig. 60, a). It is 



easy to detach individual filaments from a cord of this kind, as the 

 figure just referred to shews, in the interior of which many nuclei 

 are included, similar to those of the primitive muscular fasciculus, 

 but at a greater distance from one another. The filaments are pale, 

 granulated, and (as appears by their farther development) hollow. 

 At this period, as in muscle, a secondary deposit takes place upon 

 the inner aspect of the cell-membrane of the secondary nervous cell. 

 This secondary deposit is a fatty white-coloured substance, and it is 

 through this that the nerve acquires its opacity. This is seen 

 in fig. f,5 superiorly, at c, the fibril is still pale; inferiorly, at d, 

 the deposition of the white substance has occurred, and its 



Q 2 



Earliest 



