THE NERVOUS SYSTEM I!>1 



same state of completion, as can be done in those of other 

 situations. It is obvious, however, in the oldest Tadpoles, that 

 the nerves are somewhat thicker than they are originally, and 

 that they extend towards the periphery, sometimes in loops, 

 sometimes with free ends, but in such a way that the primary 

 pale fibres continue to exist, and, proceeding from the dark- 

 bordered fibres, constitute a very fine terminal nervous-plexus 

 with anastomoses and free ends. 



I should not have delayed so long on the subject of the 

 nerves in the Tadpole, did not similar conditions most probably 

 also obtain in many other terminations of nerves. This is 

 certain as regards those of the electric organ of the Hay, 

 which, even when developed, agree in many respects with those 

 of the more advanced Tadpole, and as Ecker has lately shown 

 ('Zeitsch. fiir wissensch. Zoologic/ 1819, p. 38), are developed 

 in precisely the same manner. The nerves, also, in the skin 

 of the Mouse (vid. note to § 1?1), evidently belong to the 

 same category; and consequently it may hereafter be shown, 

 that wherever peripheral divisions of nerves occur, their de- 

 velopment proceeds essentially as it is here described. 



With respect to the development of the nerve-fibres in the 

 central organs, we possess but few researches. Of the fibres 

 in the ganglia, I can only observe, that they are developed 

 subsequently to those of the nerves, and probably from smaller 

 fusiform cells, which mav be noticed in association with the 

 nerve-cells. On one occasion, in a spinal ganglion of a four 

 months' human embryo, I uoticed a cell of this kind in con- 

 nection with the process of a nerve-cell (fig. 164). The 

 formation of the fibres in the cord and brain is extremely 

 difficult of investigation, and is best studied with the aid of 

 chromic acid. In the human embryo, I find, as early as the 

 end of the second month, the commencement of the formation 

 of the tubules in question, the white substance being distinctly 

 finely striated, and manifestly containing, in places, very deli- 

 cate fusiform cells, which are sometimes independent or 

 isolated, sometimes connected, two or three, or several 

 together (fig. 161). All these cells are at first pale, invest- 

 ing the nucleus, which measures 0002 — 0003'" quite closely, 

 and having processes almost as fine as the fibrils of connective 

 tissue. In the fourth month, when the two kinds of substance 



