DIFFERENTIATIONS OF ECTODERM IN NECTURUS. 507 
shows the dorsal and ventral portions of the myotome parted 
by the vagus ganglion, which originally lay entirely dorsal to 
the protovertebra. Fig. 7 shows the posterior part of the 
vagus Anlage, the neural cells of which have migrated from their 
original position above the dorsal wall of the brain to their 
present position between the brain and muscle plate, where 
they finally become attached to the brain by a nerve-root. The 
difference in relation to the muscle plate between the position 
occupied by cells of the neural crest in the head, and that occu- 
pied by cells of the neural crest in the trunk, has been regarded 
as one of the essential distinctions between cranial and spinal 
ganglia. It therefore seemed of interest to note that a gan- 
glion on the border line between head and trunk develops in its 
anterior part like other cranial ganglia, and in its posterior part 
like the ganglia of the trunk. The position of the neural out- 
growth, therefore, seems to me of little value in distinguishing 
the two groups of ganglia. 
There is another peculiarity connected with the development 
of the vagus segment to which I would call attention. The 
protovertebral divisions posterior to the ear are at first of about 
the same size; but as the third vagus cleft forms in the inter- 
segment that bounds the vagus segment posteriorly, the proto- 
vertebra of this segment increases in width, and, when the 
muscle plate develops, a vertical division of the protovertebra 
occurs in a plane corresponding to the present position of the 
second vagus cleft, which, as will be remembered, has pushed 
forwards from its original intersegmental position, now occu- 
pied by the third cleft, and consequently lies beneath the vagus 
protovertebra. The two parts of the protovertebra thus severed 
are each smaller than the following protovertebre, and it is 
through the anterior of the two that the vagus root apparently 
cuts its way in the manner I have above described. 
To find a dorsal as well as ventral segmentation interpolated 
in a region where one looks for reduction and consolidation 
was unexpected. 
From the vagus segment the neural Anlage continues back- 
wards, becoming gradually reduced in size, until it consists in 
