II 



NERVE DEVELOPMENT 



111 



s.c 



above description — the existence of the motor trunk in the form of 

 a bridge of protoplasm between myotome and spinal cord at a time 

 when they are still in close proximity — has been confirmed for an- 

 other very primitive group of Gnathostomes, the Elasmobranchs 

 (Paton, 1907), as is shown in Fig. 62. 



It will now be convenient to review the facts just described for 

 Lepidosiren in relation to the general theory of nerve-development. 



(1) It is clear in the first place that the His view is put out of 

 court, seeing that before there is any development of nerve-fibrils the 

 motor nerve-trunk already exists in the form of a bridge of proto- 

 plasm connecting spinal cord and myotome. 



(2) It is equally clear that the Balfour view is inapplicable : the 

 nerve -rudiment cannot in early 



stages by any possibility be re- 

 garded as a chain of cells, seeing 

 that its total length is greatly 

 less than the diameter of a single 

 cell-nucleus. 



(3) While the nerve-rudiment 

 forms a primary connexion be- 

 tween spinal cord and myotome, 

 in the sense that it is in existence 

 before these organs begin to recede 

 from one another, there is no 

 evidence by which the connexion 

 can be traced back to intercellular 

 bridges or plasmodesms (Stras- 

 burger, 1901) of early, e.g. segmen- 

 tation, stages in the development 

 of the egg, as would be the case 

 according to Hensen's theory. 



(4) The primitive protoplasmic bridge gradually becomes fibril- 

 lated but there is no means of determining with any degree of 

 certainty how these fibrils are developed. 



It is suggested 1 that the development of the actual nerve-fibril is 

 simply the gradual coming into view of a pathway produced by the 

 repeated passage of nerve impulses over a given route. 



It is clear from the study of the simpler organisms that one of 

 the most ancient properties of living protoplasm is that of the trans- 

 mission of impulses through its substance. Although nothing is 

 really known as to the precise nature of living impulses it is reasonable 

 to suppose that they involve changes in the distribution of energy 

 analogous to those involved in the passage of an electric disturbance. 

 If this be the case their passage between two points will be determined 

 by the relative potential, and the route along which the impulse 

 passes will be that of least resistance. If the conductivity of the 



1 Graham Kerr, 1904. It has been pointed out that similar suggestions in regard 

 to the nervous system in general were made long ago by Herbert Spencer. 



Fig. 62. — Part of transverse section through 

 a 4 '5 mm. embryo of Pristiurus, show- 

 ing the motor nerve-trunk rudiment. 

 (After Stewart Paton, 1907.) 



my, myotome ; n, nerve-trunk ; s.c, spinal cord. 



