402 F. L. LANDACRE 



EXPLANATION OF FIGURES 



42 is taken five sections posterior to the auditory vesicle and lies just back of the 

 point where the root of the IX later appears. The lateral mass is here completely 

 converted into mesectoderm except the ventral portion (Y). 



43 is taken through a condensation of lateral mass cells at the point where the 

 root of the X and the jugular ganglion later appear. This mass cannot be located 

 in later series for some time. 



44 to 52 illustrate the detachment of the postauditory placode from the auditory 

 vesicle, its migration and the formation of the lateralis X ganglion. 



44 and 45 (A. neb., Stage IV) are consecutive sections. Fig. 44 being taken just 

 at the point where the auditory vesicle passes into the placode by the disappear- 

 ance of the dorsal half of the vesicle. Fig. 45 being near the anterior end of the 

 placode, three sections posterior to the auditory vesicle. 



46 is taken three sections posterior to the auditory vesicle and is from a slightly 

 less developed embryo of the same age as figs 44 and 45. The vesicle passes grad- 

 ually into the placode in this series. 



47 to 51 (A. neb., Stage VII) show the characteristic appearance of the lateralis 

 X ganglion as it is proliferated from the placode before the placode has moved be- 

 yond the posterior limit of the ganglion. Fig. 47 is twenty sections posterior to 

 the vesicle ; fig. 48 twenty-three sections ; fig. 49 twenty-five sections ; fig. 50 twenty- 

 seven sections; and fig. 51 twenty-nine sections posterior to the auditory vesicle. 

 Fig. 50 is at the extreme posterior limit of the lat. X ganglion. 



52 (A. neb., Stage IX) shows the usual appearance of the lateralis X ganglion 

 in a somewhat older series. 



53 to 60 (A. melas) illustrate the formation and detachment of theepibranchial 

 placode of the IX ganglion. 



53 (A. melas, 56* hours) shows the appearance of the placode when it can first 

 be detected. 



54 (A. melas, 69 hours) shows the extent of the thickening of the placode while 

 the whole placode is still in contact with the epidermis. 



55 to 59 are from the same embryo (A. melas, 81 hours) at a stage when the pos- 

 terior end of the placode is detached from the epidermis and has formed an attach- 

 ment to the root of the IX nerve which contains motor fibers and lateralis ganglion 

 cells. 



55, 56 and 57 are consecutive. Figs. 55 and 56 show the appearance of the gan- 

 glionic mass just before it becomes completely detached. The anterior end of the 

 ganglion is attached, for some time after the posterior end is free, apparently by 

 the growth dorsally and mesially of the ganglionic mass. 



