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 che 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 tweljih cranial or hypoglossus nerve appears on the 
fourth day as two pairs of ventral roots opposite the third and 
fourth mesoblastie 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). 
