386 F. L. LANDACRE 



loose tissue, in which the general cutaneous jugular ganglion later 

 appears, while the ventral region which forms the general visceral 

 portion of the Xth retains its continuity to a greater extent. 

 The ventral region of the lateral mass which enters into the Xth 

 ganglion resembles in appearance and corresponds in position to 

 that portion of the preauditory lateral mass which enters into the 

 geniculate ganglion. 



8. The preauditory placode which is the anterior continuation 

 of the auditory vesicle extends as far forward as the hyoid gill 

 slit, but between the ear and the hyoid gill slit it breaks down com- 

 pletely into mesectoderm. Its anterior extension disappears at 

 the exact point where the epibranchial placode of the hyoid arch 

 appears but seems to have no direct relation to it other than in its 

 position. The preauditory placode does not give rise to the pre- 

 auditory lateral line organs, there being a period of some hours 

 between the disappearance of the placode and the appearance of 

 the first lateral line organs. 



9. The postauditory placode, which is the posterior extension 

 of the auditory vesicle, becomes detached from the vesicle and 

 moves back by successive differentiations of the epidermis to 

 a point back of the fourth lateral line organ, where it gradually 

 disappears. Before its disappearance there have appeared four 

 lateral line organs anterior to it and at least two posterior to it. 

 It disappears in the region of the fifth, but probably does not give 

 rise even to that. The lateralis Xth ganglion arises by the pro- 

 liferation of cells from the postauditory placode after it has moved 

 some distance back of the auditory vesicle. After the placode 

 ceases to contribute cells to the ganglion it moves beyond the 

 posterior limit of the ganglion, losing all connection with it, and 

 does not give rise to the lateral line nerve. 



10. Both the pre- and postauditory lateral line organs are 

 formed by gradual differentiations of the deeper layer of the epi- 

 dermis, sometimes singly, sometimes two from a common pri- 

 mordium and are entirely distinct in origin from the placodes. 



11. The epibranchial ganglia all have a common mode of 

 origin. The epibranchial placode of the hyoid arch appears 

 first as a thickening of the epidermis dorsal and slightly posterior 



