DEVELOPMENT OU THE BRAIIT AND GANGLIA. 57 



eral nerves arise from these ganglion-cells. The motor nerves arise from 

 cell-groups in the gray matter, and with few exceptions leave the spinal cord 

 hy the ventral side. Most of the sensory nerves arise from ganglia which 

 lie close leside the central organ. 



From the ganglion every sensory nerve sends a number of root-fibers 

 dorsally into the central nervous system. Most of the sensory root-fibers 

 pass into the gray substance not far from the respective ganglia, but some 

 of them run some distance, forward or backward, in the superficial layers 

 before they reach their termination in the gray. These are designated as 

 ascending and descending roots. The origin and arrangement of the ganglia 

 afford much instruction and interest. 



The earliest embryonic changes show that in these peripheral structures 

 we have to deal with true derivatives of the central nervous system, — with 

 parts which separated from it qiiite early. The ganglia arise from the mar- 

 gins {Randstreifen) of the medullary plate. When this plate rises on each 

 side to form the medullary ridges which finally fuse along their marginal 

 lips to form the medullary canal, leaving on each side a free edge, these 

 margins come together and form an unpaired cell-rod, which, at first 

 wedge-shaped, appears to sink more or less into the dorsal seam of the 

 spinal cord. In consequence of an increase and a displacement of its cells it 

 soon leaves this position, leaves the roof of' the medullary tu.be completely, 

 and is divided by splitting longitudinally into a left and right cord. 

 Through segmental thickening the paired fundaments are divided into sepa- 

 rate ganglia, which move more and more laterally along the medullary tube. 

 When the segmentation is complete then the primary ganglia, so far as 

 , they are derived from elements of the central system, are complete. 



While the spinal ganglia, from their above-described derivation, re- 

 main really a part of the central system, the ganglia of the cranial segment 

 come into renewed contact with the ectoderm, or, more correctly, with the 

 embryonic epidermis for a certain period of their development, and together 

 with this form the fundaments of the primitive sense-organs. Every cranial 

 nerve acquires two such contacts, or forms fundaments of two sense-organs, 

 at two typical locations. These locations for the several cranial nerves 

 occur in two longitudinal series: one more dorsally located, the lateral, 

 or Kupjfer's, fundament, and one more ventrally located, the epibranchial, 

 or Froriep's, fundament. All of the Kupffer fundaments occur in a longi- 

 tudinal line which begins anteriorly in the olfactory pit, passes through the 

 oral pit, and in lower vertebrates is continued along the body as the lateral 

 line. All of the Proriep fundaments lie in a longitudinal line which begins 

 anteriorly in the lens-pit (Linsengrule) and thence runs along the dorsal 

 ends of all the branchial slits. Of the Kupffer fundaments two persist and 

 develop into permanent sense-organs: the nasal pit and the auditory pit. 



