58 NERVOUS SYSTEM OF VERTEBRATES. 



The ganglion extends ventrally beyond the auditory pit and comes 

 into contact with the ectoderm over the first gill slit (spiracle). 

 An ectodermal thickening fuses with the ganglion and from the 

 fused mass the rami praetrematicus and palatinus of the VII nerve 

 and the visceral sensory portion of the ramus hyomandibularis 

 take their origin. This mass is comparable to the epibranchial 

 ganglion of the IX nerve, but the significance of the ectodermal 

 constituent is equally unknown. 



The auditory pit is itself a thickening of ectoderm and it plays 

 a part in the development of certain cranial ganglia which gives 

 it the name of a dorso-lateral placode. Where the neural crest 

 anlage of the VII ganglion comes into contact with the auditory 

 pit, cells undoubtedly wander from the walls of the pit and form the 

 ganglion of the VIII nerve, no part of which is formed from the 

 neural crest. In some selachians and bony fishes (Fig. 33) an 

 ectodermal thickening continues for some distance forward and 

 backward from the auditory pit, so that we may speak of a much 

 elongated dorso-lateral placode. The cells which proliferate 

 from the caudal portion of this placode continue to multiply by 

 mitosis and form a large mass which grows and pushes backward 

 along the inner surface of the ectoderm over the roots of the IX 

 and X nerves. This mass grows caudally as a narrow band 

 along the side of the body, ploughing through the deeper layers of 

 the ectoderm. The cells derived from the placode behind the 

 auditory pit form a ganglion lying over the ganglion of the IX 

 or the first part of the X ganglion and send fibers toward the 

 brain, forming the root of the lateral line nerve which is situated 

 near that of the IX nerve as described in the last chapter. The 

 growing placode continues backward along the line of division 

 between the dorsal and the lateral body muscles and at intervals 

 gives rise to sense organs at the surface of the ectoderm. These 

 organs consist of high columnar supporting cells which form the 

 thickness of the epidermis, and shorter pear-shaped sense cells 

 which do not extend to the full depth of the epidermis but do 

 send hair-like processes out beyond the surface. The organs later 

 sink down beneath the surface so that they come to lie at the 

 bottom of the ectodermal pits (pit organs). The pits then be- 



