THE CRANIAL GANGLIA IN AMEIURUS 355 



the placode ceases to contribute cells to the ganglion the ganglion 

 ends bluntly posteriorly and there is no strand of cells connect- 

 ing the ganglion and placode. The trunk of the lateral line nerve 

 of the Xth grows out as a new structure from the ganglion. 



THE FATE OF THE POSTAUDITORY PLACODE AND THE APPEAR- 

 ANCE OF THE LATERAL LINE ORGANS OF THE BODY 



After the postauditory placode is no longer in contact with the 

 lateralis Xth ganglion it moves back some distance from the gan- 

 glion and remains approximately stationary from the fifty-sixth 

 hour up to the one hundred and thirteenth hour (A. melas), 

 during which time it gradually becomes smaller and less distinct. 

 During the greater portion of this time it can be identified posi- 

 tively both by its appearance and by its position in the body and 

 by the fact that there is nothing else with which to confuse it . In 

 the mean time there have appeared in the region traversed by this 

 placode in its backward movement and in the region from which 

 the auditory vesicle was detached from the ectoderm four lateral 

 line organs. These are the first four described by Herrick ('01, 

 plate xiv, fig. 1). The first according to his nomenclature, which 

 is innervated by the ramus oticus from the Vllth nerve, arises 

 lateral to the anterior end of the auditory vesicle and appears 

 first in my series in an embryo of 105 hours. The auditory ves- 

 icle at this time is beginning to form the semicircular canals. The 

 second organ, innervated by the ramus supratemporalis IXth, 

 develops between the posterior end of the auditory vesicle and 

 the anterior end of the lateralis Xth ganglion. It appears first in 

 the embryo of 105 hours. The third and fourth organs inner- 

 vated by twigs from the lateralis Xth ganglion develop in close 

 proximity to the posterior end of that ganglion from a common 

 primordium which elongates and gives rise to two organs. The 

 anterior portion of the common primordium gives rise to the 

 third organ and the posterior to the fourth organ. This common 

 primordium of the third and fourth organs appears first in an em- 

 bryo of 86 hours and about nineteen hours before the appearance 

 of the primordia of the first and second organs. 



