THE LATER DEVELOPMENT OF THE CHICK 313 



tirely during the sixth and seventh days. The secondary sym- 

 pathetic cords are the rudiments of the major portion of the 

 definitive sympathetic cords of the adult, and the groups of 

 fibers connecting these cords with the spinal ganglia become 

 the rami communicantes. The secondary sympathetic cords 

 become ganglionated, and additional processes grow out, con- 

 necting with the ganglia derived from the primary cords, 

 whose cell processes are finally distributed in a complex man- 

 ner to the visceral surfaces. The cardiac plexus, and the 

 plexuses of the viscera, arise chiefly from neuroblasts migrating 

 along the X nerves from the hind-brain and X ganglion. 



Two other important groups of fibers pass by way of the rami 

 communicantes; these are (1) the visceral afferent fibers be- 

 longing with the dorsal spinal root and ganglion, which pass 

 by way of the sympathetic trunks to their distribution in the 

 visceral sensory surfaces, and (2) the visceral efferent (motor) 

 fibers from the spinal cord and ventral root, to their distri- 

 bution in the visceral musculature. These latter fibers, al- 

 though arising in the cord and forming an important part of 

 the spinal nerve, are really to be regarded as components 

 of the sympathetic system. 



B. THE CRANIAL NERVES 



The cranial nerves exhibit in their development a variety that 

 parallels their diversity in morphology and in function. It is 

 possible, however, to relate many of them to parts of the typical 

 spinal nerves, and like these they have two sources, ganglia from 

 the neural crests, and neuroblasts within the spinal cord. The 

 details of their development are complicated and we may in- 

 clude here only a few of the more important facts in connection 

 with each nerve. The more posterior cranial nerves show 

 greater similarity to the spinal nerves, and as we pass forward 

 they diverge more and more widely from this type. 



The XII Cranial Nerve (Hypoglossus) . This is the nerve 

 associated with that part of the cord which has most recently 

 become included within the medulla, i.e., the region of the first 



