History of a transient nervous apparatus in certain Iclithyopsida, 411 



Occasionally one finds two such cells, directly opposite each other, 

 one in each myotome of the two sides. Such a find is interesting as 

 being an indication of a former greater symmetry of the apparatus. 

 The peripheral or vagrant ganglion-cells are usually bipolar with two 

 processes, but some give no evidence of the existence of any processes 

 whatever. Often the cells form chains reaching from the centre to, 

 and even over, the tip of the myotome. 



Transient nerves of the system. — Young embryos also 

 reveal an early formation of nerve-processes spun by the ganglion- 

 cells themselves. Especially is this true of the central ganglion-cells. 

 The process of the spinning of fibres is illustrated and described in 

 detail. All the processes tend either in the direction of the epiblast, 

 or in that of .the apex of a myotome. In the latter case they make 

 their way along its outer side lying closely under the epiblast. They 

 often, but by no means invariably, occur in pairs one on each side 

 of the body, or so nearly opposite, that a conclusion as to their 

 frequently paired character seems to find a warrant. No more than 

 any other parts of the system do these nerves manifest traces of seg- 

 mental arrangement. Usually the nerves arch over the myotome, 

 sometimes they pierce it in their course. It is a little doubtful whether 

 the final termination of any of these nerves has been determined, 

 although many of them have been followed downwards far below the 

 segmental duct. Probably some of them make their way on to the 

 yolk-sac, and this is the more probable seeing that they cease where 

 the embryo leaves the yolk-sac. 



For a long time the nerves are merely naked axis-cylinders, and 

 they are very seldom made up of more than one fibre. In later 

 stages "nuclei" occur upon them. 



Whilst most of them are undoubtedly only processes of single 

 cells, others have a more complicated build, being composed of 

 a plexiform arrangement of ganglion-cells and nerve-processes. Often 

 there occur ganglion- cells , applied end to end, in the course of the 

 nerve, still more frequently there is a coating of such cells upon 

 the fibre. ^ 



In the build of these nerves all sorts of bewildering variations 

 are met with in different embryos. 



As to the functions of these nerves the conclusion now drawn is 

 that it is a sensory one, and this is held to be true of the whole 

 apparatus. 



The system as described is often well-developed in embryos of 



