354 ORGANIC GROWTH sec. 



of that tissue, connected by definite but extremely primitive 

 nerve-fibres, form chains passing through the tissue from one 

 surface to the other. I believe that we have here a survival 

 of the most primitive nervous communications in multicellular 

 animals. The course of nervous impulses, originally variable, 

 has probably become fixed in consequence of their continual 

 passage in the same direction ; and from the same cause, to 

 judge from the relations above described of neuro-ectodermic- 

 cells, has been developed the axis-cylinder, composed of fine 

 fibrils, of the nerves of higher animals. 



The nerves of the cycloneurous Medusae, which are also 

 chains of nerve-cells, are, on the other hand, developed in such 

 a way that they remain dividing ectoderm-cells connected 

 from the first by nerve-fibrils, which are processes from them- 

 selves. Thus arise the ganglion-cells interpolated in these 

 animals in the course of nerve-fibres. 



When making experiments on the section of Medusae, I 

 made an observation on the vicarious action of nerve-fibres, 

 which finds its proper place here. I was trying at the time 

 to ascertain whether in the Scyphomedusae (toponeurous 

 Medusae) a nerve (ring-nerve) is present at or near the 

 margin of the umbrella. With this object I made a cut in an 

 Aurelia aurita, with scissors, through the margin for a distance 

 of 1 cm. between every two marginal bodies, in order to see 

 whether the interdependence of the movements of the eight 

 portions of the umbrella on one another, in other words, 

 whether the co-ordination of the motion excited by the 

 activity of the several ganglia, would continue or not. As the 

 co-ordination was completely maintained, I next, in another 

 Aurelia, cut out the whole central portion, leaving a 

 continuous marginal ring about \\ cm. broad, and contain- 

 ing all the eight marginal bodies.^ This ring began, after 

 a pause such as always occurred after similar operations 



^ The following figure is also taken from the Medusae, p, 31. 



