EXPLANATION OF FIGURES

34 is taken through the anterior end of the dorso-lateral lateralis VII ganglion and the root of the Gasserian.

35 is taken through the trunk (hyomandibular nerve) of the ventral lateralis ganglion of the VII and the root of the geniculate ganglion.

36 is taken through the root of the ventral lateralis ganglion.

37 is taken through the extreme anterior tip of the auditory ganglion. The dorsal lateralis VII runs through figs. 34 to 37.

38 and 39 are taken through the anterior and median portions of the auditory ganglion. The roots of the dorso-lateral, ventro-lateral and geniculate ganglia appear in figs. 37, 38 and 39.

40 to 43 (A. neb., Stage VII) are taken from the same embryo as figs. 25 to 30. Figs. 40 and 41 are consecutive sections and show the only recognizable trace of a condensation of the cells derived from the lateral mass in the region of the IX nerve, except the ventral portion of the lateral mass (Y) which does not enter into the IX ganglion.

40 is taken one section posterior to the auditory vesicle. 41 is taken two sections posterior to the auditory vesicle.

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 disappearance 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 gradually 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 beyond 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 twentyseven 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 the epibranchial 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 posterior end of the placode is detached from the epidermis and has formed an attachment 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 ganglionic 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.

EXPLANATION OF FIGURES

57 is taken one section posterior to fig. 56 at the point where the ganglionic mass is free from the epidermis.

58 and 59 are taken through the root of the epibranchial ganglion of the IXth just before it comes into contact with the lateralis and motor portions of the IX, as shown in fig. 59.

60 (A. melas, 93 hours) is taken through the epibranchial ganglion at the nearest point of approach to the epidermis. This is the first stage in which the ganglion is completely detached from the epidermis. Its clean cut boundaries during all of its development indicate that no other cells than those derived from the placode are incorporated in the ganglion.

61, 62 and 63 (A. neb.) illustrate three stages in the formation of the lateralis IX ganglion of which fig. 63 (A. neb., Stage V) is the earliest. The ganglion varies a great deal in appearance and somewhat in position. Fig. 62 (A. neb., Stage IX) is older than fig. 61 (A. neb., Stage VIII). Fig. 63, while taken from the posterior end of the vesicle, differs totally from the appearance of this region before and after the formation of the ganglion.

64 to 71 show the formation of the first two epibranchial ganglia of the X.

64 (A. melas) 69 hours shows the earliest recognizable trace of the third epibranchial placode. Figs. 65 to 71 are from the same embryo (A. melas, 93 hours). 65 is through the anterior portion of the placode.

66 is through the middle region just before the detached portion is reached.

67 is just backof the point of detachment.

68 is through the root of the third epibranchial ganglion and the fourth epibranchial placode. This probably is not the earliest trace of the fourth epibranchial placode, however. The preceding series is defective at this point.