460 EMBRYOLOGY. 



As is evident from this brief survey, there still exist many unsolved 

 problems in the difficult subject of the development of the peripheral 

 nervous system. Without permitting myself to enter upon a further 

 discussion of the contradictory opinions entertained on this subject, 

 I close this topic with a comparative-anatomical proposition, which 

 appears to me sufficient to furnish the morphological explanation of 

 BELL'S law, or the separate origin of the sensory and motor nerve- 

 roots. 



In Arnphioxus and the Gyclostomes the motor and sensory nerve- 

 fibres are completely separated, not only at their origin from the 

 spinal cord, but also throughout their whole peripheral distribution. 

 The former pass at once from their origin in the spinal cord to the 

 muscle-segments ; the latter ascend to the surface to be distributed 

 to all parts of the skin to supply its sensory cells and sensory organs. 

 The separation of the peripheral nervous system into a sensory and a 

 motor portion, ivhich is rigorously carried out in Amphioxus and the 

 Gyclostomes, is explained by the fact that the territories to which their 

 ends are distributed are spatially distinct in their origin, since the 

 sensory cells arise from the outer germ-layer, the voluntary muscles 

 from a tract of the middle germ-layer. Therefore the sensory nerve- 

 fibres have been developed from the spinal cord in connection with the 

 outer germ-layer, the motor fibres in relation with the muscle- 

 segments. 



I regard the sub-epithelial position of the sensory nerve-fibres as 

 the original one, just as we find in many Invertebrates the whole 

 peripheral sensory nervous system developed as a plexus in the 

 deepest portion of the epidermis. The important conditions above 

 described according to which many dermal nerves (nervus lateralis, 

 etc., fig. 262 nl) are fused with the epidermis at the time of their 

 origin, and only subsequently become detached from it and sink 

 deeper into the underlying mesenchyme appear to me to indicate 

 that such a position was the primitive one in the case of Vertebrates 

 also. 



I look upon the union of the sensory and motor nerve-fibres into 

 mixed trunks (which occurs soon after their separate origin from 

 the spinal cord, in the case of all Vertebrates except Amphioxus and 

 the Cyclostomes) as a secondary condition, and maintain that it is 

 caused especially by the following embryological influences : by the 

 change in the position of the spinal cord and the muscular 

 masses, and by the great increase in the amount of the connective 

 substances. 



