140 OUTLINES OF CHORDATE DEVELOPMENT 



riorly, through the dorsal head region, and also medially con- 

 necting with the medulla. The posterior part of the placode 

 ganglion then fuses with the crest ganglion and together they 

 form the trigeminal ganglion proper (Gasserian ganglion). 

 From the neuroblasts of this ganglion, cell processes extend 

 centripetally, entering the dorsal side of the medulla and form- 

 ing the sensory root of the V nerve, while centrifugal processes 

 rapidly grow out to the surface of the head (cutaneous branch of 

 the V nerve), and also in front of and behind the mouth 

 (mandibular and maxillary branches). These branches, as 

 indeed those of most of the other branchiomeric nerves as 

 well, are established before the time of the opening of the mouth. 

 The cells which formed the original attachment of the crest 

 ganglion with the medulla, appear to form the medullary sheath 

 cells of the root of this nerve, while the sheaths of the peripheral 

 branches are apparently derived from other migratory cells of 

 the crest ganglion itself. 



The Facial and Auditory Nerves (VII and VIII). It is con- 

 venient to describe these together, since both are derived from 

 the acustico-facialis crest ganglion and its associated placode. 

 The VII nerve is the nerve of the hyo-mandibular cleft, while 

 the VIII nerve is not one of the branchiomeric series, but is 

 purely sensory (auditory) . 



The early history of these nerves is similar to that of V; the 

 major part of the crest ganglion contributes to the mesenchyme 

 of the hyoid arch. The nervous part of the crest ganglion is 

 somewhat more extensive than that of the V nerve, i.e., a greater 

 part of the original ganglion remains of nervous function. 

 The first important distinctive character here, consists in the 

 fact that the sensory or superficial part of the placode does not 

 disappear, but, continuing to enlarge, gradually sinks below the 

 surface of the head, invaginates, and forms the rudiment of 

 the ear, the auditory sac (Fig. 45, C). The deeper placode 

 ganglion cells in connection with this sensory epithelium remain 

 in close contact with the sac forming the rudiment of the VIII 

 nerve. From the neuroblasts of this rudiment processes grow 

 into the medulla forming the root of the VIII nerve. 



