THE LATER DEVELOPMENT OF THE CHICK 315 



of the common acustico-facialis ganglion becomes intimately 

 related with the rudiment of the ear (auditory sac) and later 

 differentiates in situ. It has no efferent fibers equivalent to 

 the ventral spinal roots or to the efferent components of the 

 cranial nerves so far described. 



The VII Cranial Nerve (Facial). That portion of the acus- 

 tico-facialis ganglion remaining after the differentiation of the 

 VIII ganglion and nerve, remains as the geniculate ganglion 

 of the VII nerve. This connects with the medulla in the usual 

 fashion, and distally fibers grow out into the vicinity of the 

 first or hyomandibular visceral pouch (spiracle) with which 

 this nerve is primarily related. In the chick it is chiefly 

 visceral efferent, but the development of these motor compo- 

 nents is not known, although the cells composing its nuclei of 

 origin in the medulla, have the usual position relative to other 

 centers. 



The V Cranial Nerve (Trigeminal) . The trigeminal ganglion 

 derived from the neural crest connects with the medulla in the 

 usual way and early becomes partially divided. From the 

 smaller anterior portion (profundus ganglion) fibers grow for- 

 ward as the deep ophthalmic branch, while from the posterior 

 part (trigeminal ganglion proper) arise the branches distributed 

 to the upper and lower jaws. This nerve is chiefly somatic 

 afferent, its small efferent component developing typically, 

 during the fourth day. 



The IV and VI Cranial Nerves (Trochlear and Abducent). 

 These two nerves have many characteristics in common. 

 They are purely motor, distributed to muscles of the eye-ball 

 (superior oblique and external rectus, respectively), and con- 

 sequently have no ganglia outside the medulla. They appear 

 relatively late (fourth day), the IV from the dorsal surface of the 

 isthmus, the VI from the ventral side of the myelencephalon. 



The III Cranial Nerve (Oculomotor). This too is chiefly 

 motor, supplying the remaining muscles of the eye-ball. Its 

 fibers grow out from the ventral side of the mid- brain and 

 extend into the mesenchyme around the developing eye. 

 Associated with its motor elements are afferent components 



