262 THE DEVELOPMENT OF THE CHICK 
epithelium, and the optic is really comparable to an intramedul- 
lary nerve tract, seeing that its termination lies in a part of the 
original wall of the neural tube, viz., the retina. 
Groups of medullary neuroblasts giving rise to axones of 
motor cranial nerves are located in the brain as follows, according 
to His: 
Oculo-motor nucleus in the mid-brain. 
Trochlearis nucleus in the isthmus. 
Motor trigeminus nucleus in the zone of the cerebellum, including 
the descending root. 
Abducens and facialis nuclei, beyond zone of greatest width 
of the fourth ventricle (auditory sac zone). 
Glossopharyngeus, vagus, in the region of the calamus_ scrip- 
torius. 
Accessorius and hypoglossus, in the region extending to the 
cervical flexure. 
These constitute the cranial motor nerve nuclei, and are more 
or less discontinuous. 
The ganglionic nerves or nerve-components of the head arise 
from the following primitive embryonic ganglion-complexes: 
1. Complex of the trigeminus ganglia. 
2. Complex of the acustico-facialis ganglia. 
3. Complex of the glossopharyngeus ganglia. 
4. Complex of the vagus ganglia. 
The early history of these ganglion-complexes has already been 
considered; they are called complexes because each forms more 
than one definitive ganghon. It is probable also that each con- 
tains sympathetic neuroblasts, which may separate out later as dis- 
tinct ganglia, thus resembling the spinal sympathetic neuroblasts. 
There is no close agreement in the segmentation of the motor 
neuroblasts within the brain and that of the ganglion complexes. 
For instance, in the region of the trigeminal ganglionic complex, 
the motor nuclei of the oculo-motor, trochlearis, and trigeminus 
are found, and in the region of the vagus ganglionic complex, 
the motor nuclei of vagus, accessorius, and hypoglossus. Thus 
the medullary and ganghonic nerves of the head are primitively 
separate by virtue of their separate origins. They may remain 
entirely so, as in the case of the olfactory, trochlearis, and abdu- 
cens, or they may unite in the most varied manners to form 
mixed nerves. 
