GANGLIA AND NERVES OF SQUALUS 41 
tral to this muscle primordium. The epibranchial portions of 
X, and X; which are still attached to the ectoderm are more 
lateral in position and not directly under the muscle primordium. 
The muscle primordium is pierced by the proximal end of the 
visceral ganglion of branchial Xo». 
22. THE VAGUS ROOTS 
The analysis of the vagus roots in detail beyond the number 
and point of attachment is very difficult and sometimes impos- 
sible in a 22 mm. embryo. 
The first root of X (fig. 1, Rt.L.IJX-+X) is a lateral line root 
and enters along with the lateral line root of LX just dorsal to 
the visceral sensory and motor roots of IX. Posterior to this 
most anterior root are three roots much alike in appearance. 
These three roots arise anterior to the level of the point of origin 
of the r. supratemporalis X. The second, third and fourth 
roots arise from the thicker anterior portion of the X ganglion, 
while the first or lateralis root, joins the brain wall only after 
a rather long course cephalad as is usually the case with the 
lateral line root of X which connects the X ganglion with the IX. 
Each of the second, third and fourth roots is round in trans- 
verse section, contains a rather large number of cells on the 
anterior face of the root, and as it enters the brain wall, divides 
into two divisions one of which turns slightly dorsally and the 
other ventrally and mesially. Owing to the very minute size 
of the general cutaneous rami of X the dorsal division is identi- 
fied provisionally as visceral sensory and the more ventral divi- 
sion as visceral motor. The more dorsal fibers do not enter the 
tract which the lateral line fibers of IX and X from the first 
root enter, but do enter the same column entered by the more 
ventral or visceral root of IX. Caudally of the first four roots 
there are twelve to fifteen roots (not named on figure 1) arising 
from the more attenuated caudal portion of the ganglion, all of 
which show the same composition as the second, third and fourth. 
They are slightly smaller and each root on entering the medulla 
divides into a dorsal and a ventral branch. The dorsal division 
becomes progressively smaller in the more posterior roots, and 
