SECT. I25.] NERVOUS SYSTEM. 259 



1846, p. 102, tab. 6, 7), be traced with facility in the tail of the 

 larva of the naked amphibia (fig. 107, 3; 108). Here we find, as 

 the first rudiments of the nerves, pale branched fibres, o - ooi"' to 

 0'002'" in diameter, which anastomose at certain places, and finally 

 terminate in extremely fine fibrils, o - ooo2'" to o - ooo-j/" in diameter. 

 There is iiot the slightest difficulty in showing that these fibres 

 arise by the coalescence of fusiform or stellated cells ; for, firstly, 

 we see such cells partly lying close to the fibres, but distinct from 

 them, partly more or less connected with their off-shoots ; and, 

 secondly, we find, in the somewhat swollen places of division of 

 the fibres, distinct cell-nuclei, and with them, at least in young 

 larvae, the well-known angular yelk granules, which originally 

 fill all the cells of the embryo. At first the pale embryonic nerves 

 are very few in number, and are limited to some short trunks lying 

 close to the muscles of the tail; gradually, however, they are 

 developed in a peripheral direction farther into the transparent 

 parts of the tail, and in such a manner, that new cells continually 

 become united with the nerve-trunks already present, "whilst 

 the latter unite with each other by means of slender off-shoots, 

 almost like the capillaries of the same animal. These ramifica- 

 tions, when once formed, undergo further changes, as follows : 

 Whilst the fibres gradually thicken to twice or four times their 

 original diameter, dark-bordered fine primitive fibres are gra- 

 dually developed in them, in the direction from the trunks to the 

 branches, which, in no case, owe their origin to any newly super- 

 added medullary sheaths, but are certainly formed only by the 

 metamorphosis of a part of the contents of the pale fibres. The 

 following relations, which have not hitherto been seen in the 

 higher animals, are worthy of note: 1. The dark-bordered tubes 

 scarcely ever entirely fill the pale fibres in which they arise, there 

 being generally an interspace, often as wide as the fibres them- 

 selves, remaining between them and the sheath of the embryonic 

 fibres, in which the nuclei of the primitive formative cells may 

 occasionally be seen. 2. In the trunks and main branches, it is 

 undoubted that several (2 — 4) dark-bordered tubes become de- 

 veloped within one and the same embryonic fibre; a very remark- 

 able fact, proving the existence of dark-bordered tubes without 

 structureless sheaths, and showing a correspondence with the 

 muscular fasciculi, in which, likewise, many fine elements arise 

 within one tube. Since the tails of tadpoles fall off at a subse- 

 quent period, unfortunately their interesting nerves cannot be 

 followed to such a complete stage of development, as those of 



s 2 



