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 ganglion. It is probable also that each con- 

 tains sympathetic neuroblasts, which may separate out later as dis- 

 tinct ganglia, thus reseml)ling 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 ganglionic nerves of the head are primitively 

 separate by virtue of their separate origins. They may remain 

 entirelv 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. 



