THE NERVOUS SYSTEM. 429 



ciated the well-known angular vitelline corpuscles, witli which at first, 

 all the cells of the embryo are filled. At first the number of pale 

 embryonic nerves is very small, and limited to a few short trunks closely 

 applied to the muscular structures in the tail ; but they are gradually 

 developed, in the direction from the centre towards the periphery, fur- 

 ther into the transparent portion of the tail, new cells being continually 

 added in connection with the existing trunks, whilst the latter them- 

 selves, almost in the same manner as the capillaries of these larvae, 

 unite directly by delicate offsets. When these fine ramifications — with 

 respect to the nervous nature of which no doubt can be entertained, as 

 it is evident that the larvae in which they exist already possess very 

 acute sensibility — are once formed, the followinsr further changes then 

 take place. Whilst the fibres gradually enlarge to twice or four times 

 their orginal diameter, there are by degrees developed in them, and in 

 fact from the trunk towards the branches, dark-bordered, fine primitive 

 fibres, which in no case owe their origin to newly added medullary 

 sheaths, but are certainly formed solely from a metamorphosis of a part 

 of the contents of the pale fibres. In connection with this, however, 

 the following conditions, which have not yet been observed in the higher 

 animals, are to be remarked: 1, where a pale embryonic fibre bifurcates, 

 there occasionally, though not always, also takes place a division of the 

 dark-bordered tube developed within it ; 2, the dark-bordered tubes 

 scarcely ever completely fill the pale fibres in which they are formed, 

 but a space, frequently of the same diameter as that of the tubes, is 

 most usually left between them and the membranes of the embryonic 

 fibres, in which space occasionally the nuclei of the primordial formative 

 cells may be perceived ; 3, in the trunks and main branches of the 

 embryonic fibres, several (2-4) dark-bordered tubules are undoubtedly 

 developed within one and the same embryonic fibre ; a very remarkable 

 condition, which shows that there are even dark-bordered fibres which do 

 not possess a structureless sheath {yid. note to § 110), and resembling 

 what exists in the muscular fasciculus, in which, in like manner, within a 

 single tubule, numerous finer elements are produced. As the tail of 

 the Tadpole is afterwards thrown off, it is to be regretted that its in- 

 teresting nerves cannot be traced to the 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 



