462 EMBRYOLOGY. 



Since the spinal cord comes to lie in deeper layers of the body 

 far away from its place of origin, the dermal nerves must follow it, 

 and therefore their origins are correspondingly farther separated 

 from their terminations. Since also, on the other hand, the muscle- 

 plates grow around the neural tube, certain motor and sensory 

 nerve-cords are brought near to each other in their passage to their 

 peripheral distribution. And this will occur especially in all cases 

 where the motor and sensory peripheral terminations lie at a great 

 distance from the origin of the nerves out of the spinal cord, as, for 

 example, in the case of the limbs. The mutual approximation of 

 sensory and motor nerve-tracts thus brought about will finally lead 

 to the formation of common tracts, according to the same principle 

 of simplified organisation in accordance with which the blood-vessels 

 also adapt themselves closely to the course of the nerves. 



(c) The Development of the Sympathetic System. 



The development of the sympathetic nervous system has as yet 

 been investigated by only a few observers. BALFOUR first announced 

 that it arose in connection with the cranial and spinal nerves, and 

 therefore was, like the latter, really derived from the outer germ- 

 layer. In the Selachians he found the sympathetic ganglia (fig. 262 

 sy.y] as small enlargements of the chief trunks of the spinal 

 nerves (sp.n) a little below their ganglia (s/>.#). In older embryos, 

 according to BALFOUR'S account, they recede from the spinal 

 ganglia, and then at a later period unite with one another, by the 

 development of a longitudinal commissure, into a continuous cord 

 (Grenzstrang). 



The origin of the sympathetic system has been the most thoroughly 

 studied by ONODI in researches covering several classes of Verte- 

 brates. According to him the sympathetic ganglia arise directly, as 

 BALFOUR suggested and as BEARD has also lately reiterated, from the 

 spinal ganglia. The ventral ends of the spinal ganglia undergo 

 proliferation, as is best seen in Fishes. The proliferated part de- 

 taches itself, and, as fundament of a sympathetic ganglion, moves 

 ventrally. The fundaments of the individual segments are at first 

 separate from one another. The cord (Grenzstrang) is a secondary 

 product, produced by the growing out of the individual ganglia 

 toward each other and the union of the outgrowths. Afterwards 

 the sympathetic ganglia and plexuses of the body-cavity are derived 

 from this part. 



