THE CRANIAL GANGLIA IN AMEIURUS 345 



it passes gradually into the epidermis here, while on its dorsal 

 surface it passes suddenly into rather thin epidermis. The ap- 

 pearance in this section indicates that the cells contributed to the 

 ganglion before its detachment come mainly from the epidermis 

 situated ventrally to its point of attachment. This conclusion 

 is verified from a study of the epibranchial placodes of the IXth 

 and Xth ganglia. The whole of the ganglion shown in fig. 29 

 is not derived from the placode. The portion situated mesially 

 and dorsally (L. M . G. VII, fig. 29) is derived from the remains 

 of the lateral mass lying anterior to the lateralis Vllth ganglion. 

 The separation between these two constituents of the ganglion 

 is faintly indicated in the figure. The presence of these lateral 

 mass cells can be first detected in the section preceding the one 

 from which fig. 29 is drawn and the separation is there somewhat 

 more distinct than in the section sketched. In the section fol- 

 lowing the one from which fig. 29 was taken (fig. 30) the division 

 between the two constituents is quite apparent and the portion 

 of the ganglion derived from the lateral mass is slightly larger 

 than the placodal constituent. A few sections posterior to this 

 point the placodal portion of the ganglion ceases to be present, 

 while the lateral mass portion reaches its maximum size in this 

 embryo. 



The posterior end of the lateral mass portion of the ganglion 

 cannot be definitely determined, since at this stage it simply 

 becomes looser in texture and finally before reaching the region 

 of the lateralis Vllth can no longer be distinguished from that 

 ganglion and from the ventrally situated mesoderm. The fact 

 that this ganglionic mass is at its posterior end rather closely 

 applied to the ventrally situated mesoderm makes it impossible 

 to determine the exact boundary posteriorly before the ganglion 

 assumes definite shape, as it does a little later after the detachment 

 of the placodal portion; in addition to this fact the fibrillated 

 root of the whole ganglionic mass appears at the posterior end of 

 the ganglion and the point where this appears is in most of the 

 cranial ganglia preceded by a more or less ill-defined mass of cells 

 which renders the determination of exact boundaries difficult. 



THE JOURNAL OP COMPARATIVE NEUROLOGY AND PSYCHOLOGY, VOL. 20, NO. 4. 



