STOMODAEU.W AND MEDULLARY TUBE 15 



daeum, and in this way the latter might have been taken up into 

 the nervous system so as to form part of it. With such a sup- 

 position the facts of ontogeny are in harmony. As His (1886) 

 first observed in the human embryo, the dorsal nerve roots are 

 produced exclusively by the spinal ganglia. Fro n these grow 

 out the nerve-fibres which const tute the peripheral part of 

 the dorsal nerve, whereas on the other side they grow out into 

 the medullary tube, thus constituting the root of the dorsal 

 nerve. With this result, those obtained experimentally 

 by cutting off the dorsal nerve under and above the ganglion 

 and studying the degeneration of the nerve fibres, as shown 

 first by WALLER (1852) and afterwards by many others, 

 are in harmony. It can be demonstrated in this way that 

 the ganglion is the trophic centre for the whole dorsal root. 

 Finally it may be observed that the sympathetic nervous 

 system with its ganglia is a derivative of the spinal ganglia. 



Spinal nerves. — If now the assumption made above is 

 correct the dorsal component of the spinal nerves issuing 

 from the spinal ganglion must be compared with the seg- 

 mental nerves radiating from the ventral ganglion of the 

 Annelid. These latter nerves, however, are of mixed motor 

 and sensory function, and thus we are led to a conclusion, 

 put forward for Vertebrates by FRANCIS BALFOUR as early 

 as 1878. Balfour did not observe ventral roots in Amphioxus 

 and the dorsal nerves are of a mixed sensory and motor 

 nature. The same applies to the dorsal nerves of the head 

 of Craniotes, where also BALFOUR did not recognize any 

 ventral roots. Thus he expressed as his opinon (I.e. p. 193): 

 "that primitively the cranio-spinal nerves of Vertebrates 

 were nerves of mixed function with one root only, and 

 that root a dorsal one, and that the present anterior or 

 ventral root is a secondary acquisition" (cf. also 1881, II, 

 p. 382). The original condition is found still in Amphioxus 

 and in the head of Craniates 



To this conclusion VAN Wyhe (1882, p. 40) has objected : 



P. that soon afterwards ventral roots have been dis- 

 covered in Amphioxus (SCHNEIDER, 1879, p. 15), 



2**. that in the trunk the dorsal root originates indepen- 

 dently of the ventral one. VAN WYHE here evidently 

 assumes that in Balfour's opinion the ventral nerves are 

 to be derived from the dorsal ones, 



3''. thai Balfour did not consider the oculomotorius, 

 Irochlearis and abducens as ventral roots, as proposed by 



