THE LATER DEVELOPMENT OF THE CHICK 311 



around the first visceral pouch, and appear to receive cells pro- 

 liferated from this layer. This connection is not to be con- 

 fused with the much later connections established by the VII, 

 IX, and X ganglia with the epibranchial placodes. 



3. The Peripheral Nervous System 



We must distinguish here between the spinal and cranial 

 nerves. The spinal nerves, of which there are some thirty- 

 eight pairs distinguishable at certain stages, all develop essen- 

 tially alike, while the twelve pairs of cranial nerves may differ 

 widely form one another and from the spinal nerves, in their 

 development, as they do in morphology and in function. 



A. THE SPINAL NERVES AND THE SYMPATHETIC SYSTEM 



Each spinal nerve is a complex, including parts of unlike 

 function and relation to other structures. For our present 

 purposes it will be necessary to distinguish only the dorsal or 

 afferent and the ventral or efferent portions, and in each of these 

 the somatic (somatopleural) and splanchnic (splanchnopleural) 

 pathways. Brief reference to the sympathetic system may be 

 included here because it is obviously derived, to a certain 

 extent, from the same rudiments as are the spinal nerves. 



The rudiments of the somatic afferent portions of the spinal 

 nerves are the cell-groups derived from the neural crests de- 

 scribed above, i.e., the spinal ganglia. During the third day 

 the neuroblasts of the spinal ganglia send out processes (axons, 

 axis cylinder processes) in two directions, centripetally into the 

 spinal cord, and centrifugally, meeting and joining the ventral 

 root; the cell bodies thus remain as the spinal ganglion cells. 

 The ventral root of each spinal nerve is efferent and is composed 

 of outgrowths (axons) from both somatic and splanchnic neuro- 

 blasts located in certain regions of the ventral cornua of the 

 spinal cord. The dorsal and ventral (afferent and efferent) 

 roots unite a short distance from the cord forming the spinal 

 nerve trunk. This trunk then immediately divides into three 



