Embryology of certain of the Lower Fishes. 207 



development, when it is found in the form of a soft proto- 

 plasmic bridge connecting spinal cord and myotome, the 

 latter structures being still in contact with one another. 

 As development proceeds, the myotome becomes pushed out- 

 wards by the development of mesenchyme, and the nerve- 

 trunk becomes correspondingly lengthened. As the myotome 

 increases in size, the peripheral end of the nerve becomes 

 expanded, and this expansion breaks up into conically 

 arranged branches. Later on the myotome breaks up into 

 various muscles, which become pushed about into their adult 

 positions, each one dragging with it its bit of the original 

 nerve-trunk to form its nerve in the adult state. The nerve- 

 trunk is, as I have shown, originally naked ; its sheath is 

 derived from mesenchymatous elements, which apply them- 

 selves to it. This spreads itself out along the nerve, and 

 forms the continuous protoplasmic sheath. The conducting 

 elements of the nerve, the primitive fibrils, gradually appear 

 in the originally simple granular protoplasm of the nerve- 

 trunk. 



The special interest of these phenomena of nerve develop- 

 ment lies in their bearing on the general problem. It will 

 be seen that they render quite unnecessary the remarkable 

 view, which is still taught by many in authority, that nerve- 

 trunks grow out towards the periphery. Evidence to sup- 

 port this view is, of course, plentiful enough: it may be 

 found in almost every slide of sections. It would have been 

 easy to furnish numerous accurate figures of sections show- 

 ing freely ending nerves ; while it needed much laborious 

 searching through long series of sections of carefully fixed 

 and prepared embryos before it was possible to get together 

 a set of specimens like those which I have figured, showing 

 the whole course of the nerve-trunk. It requires very little 

 consideration to realise. that one must frequently find what 

 seem to be free ending nerves, not because they really are so, 

 but because the nerve passes out of the plane of section. 

 It is often very difficult indeed to pick up such nerves in the 

 neighbouring sections of the series. There is also the further 

 possibility that the delicate and inconspicuous nerve-trunk 

 may not be towards its peripheral end still naked, and the 



VOL. XVI. T 



