THE CRANIAL GANGLIA IN AMEIURUS 353 



are separated in the head. In the more generalized Xth the 

 general cutaneous is intra-cranial and the general visceral extra- 

 cranial, while in the more highly specialized region of the Vth 

 and Vllth both are intra-cranial. 



THE ORIGIN OF THE LATERALIS Xth AND THE EARLY STAGES OF 

 THE POSTAUDITORY PLACODE 



The lateralis Xth is derived not from the lateral mass but from 

 the postauditory placode as it moves away from the auditory 

 vesicle. The first evidence I find of a separation between the 

 postauditory placode and the auditory vesicle is shown in fig. 44, 

 Stage IV. The dorsal portion of the auditory pit was present 

 in the preceding section. In fig. 44 only the ventral portion re- 

 mains as the placode. The portion which disappears is that most 

 nearly in contact with the medulla. In the following section 

 (fig. 45) the placode has lost all connection with the dorso-mesial 

 portion of the lateral mass which is beginning to be converted into 

 mesectoderm. The length of the placode at this time is only 

 four sections. Fig. 46, Stage IV, shows the appearance of the 

 placode in a series of the same age, but less developed, in which 

 the auditory vesicle is continued into the placode with no break 

 in continuity. The placode is here seven sections long. 



The placode now moves back apparently after it is detached 

 from the auditory vesicle. My series is incomplete at this period 

 and I am consequently unable to describe the earliest appearance 

 of the anterior end of the lateralis Xth or the rate at which the 

 placode moves away from the auditory vesicle at first. At the 

 time, however, when the anterior end of the placode has reached 

 a distance of twenty-one sections from the vesicle, Stage VII, the 

 placode has not changed in appearance but the lateralis Xth gan- 

 glion is present and has a length of ten sections and overlaps the 

 placode for five sections. Anterior to the region of overlap the 

 lateralis ganglion lies just under the epidermis between it and the 

 mesectoderm as a rather irregular mass of cells (fig. 47) . Through- 

 out the whole region of the overlap the placode is contributing 



