490 SPECIAL HISTOLOGY. 



with them are associated the well-known angular vitelline 

 corpuscles, with 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 de- 

 veloped, in the direction from the centre towards the periphery, 

 further into the transparent portion of the tail, new cells being 

 continually added in connection with the existing trunks, 

 whilst the latter themselves, almost in the same manner as 

 the capillaries of these larvae, unite directly by delicate off-sets. 

 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 following further changes 

 then take place. Whilst the fibres gradually enlarge to twice 

 or four times their original 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 (vid. note to § 110), and resembling 

 w 7 hat 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 interesting nerves cannot be traced to the 



