THE NERVOUS SYSTEM 269 



nerve passes ventrally and enters the ganglion nodosum situated 

 above the fourth and fifth visceral arches. Branches pass from 

 here into the fourth and fifth arches, and the main stem is con- 

 tinued backward as the pneumogastric nerve s.s. From the hinder 

 portion of the spreading roots a strong commissure is continued 

 backward parallel to and near the base of the neural tube as far 

 as the fifth somite; this is provided with three small ganglion-like 

 swellings. This condition is found about the end of the fourth 

 day. Later this commissure unites with the main sympathetic 

 trunk, and part of the vagus ganglion separates from the remain- 

 der as the ganglion cervicale primum of the sympathetic trunk. 



During the fifth and sixth days the main stem of the vagus 

 grows farther back and innervates the heart, lungs, and stomach. 

 Neuroblasts of the sympathetic system accompany the vagus 

 in its growth, and form the various ganglion cells of the heart, 

 and other organs innervated by the vagus. 



During the fifth and sixth days the ganglion nodosum, which 

 originally lay at the hind end of the pharynx, is carried down 

 with the retreat of the heart into the thorax, and on the eighth 

 day it is situated at the base of the neck in close contact with 

 the thymus gland. 



11. The Eleventh Cranial or Spinal Accessory Nerve. No ob- 

 servations on the development of this nerve in the chick are 

 known to me. 



12. The twelfth cranial or hypoglossus nerve appears on the 

 fourth day as two pairs of ventral roots opposite the third and 

 fourth mesoblastic somites; each root is formed, like the ventral 

 roots of the spinal nerves, of several bundles that unite in a com- 

 mon slender trunk ; ganglia are lacking, as in the first and second 

 cervical nerves. The roots of the hypoglossus are a direct con- 

 tinuation of the series of ventral spinal roots, and as they are 

 related to somitic muscle plates in the same way as the latter, 

 there can be no doubt of their serial homology with ventral roots 

 of spinal nerves. The first four mesoblastic somites are subse- 

 quently incorporated in the occipital region of the skull, and 

 thus the hypoglossus nerve becomes a cranial nerve. No nerves 

 are formed in connection with the first and second mesoblastic 

 somites. As the occipital region of the skull forms in the region 

 of the occipital somites, two foramina are left on each side for 

 exit of the roots of the hypoglossus (Figs. 150 and 244). 



