380 F. L. LANDACRfe 



The embryological evidence based on the mode of origin of the 

 Xth ganglion strongly supports Herrick's suggestion, based on a 

 study of the adult condition in Menidia, since the general and 

 special visceral ganglionic cells come from two different sources 

 although combined into one ganglionic mass. 



The intra-cranial or jugular ganglion is the last of the cranial 

 ganglia to assume definite form, and can be recognized first as 

 a definitive ganglionic mass in an embryo of 113 hours. At this 

 time the backward growth of the ear has carried the lateralis 

 IXth quite near to the future jugular ganglion, but they are sep- 

 arated here as in all preceding stages by a blood vessel. Between 

 the 69-hour stage and the 113-hour stage the jugular ganglion 

 is not definitely formed and appears gradually as a mass of cells 

 surrounding the root of the Xth nerve and extending from near 

 the medulla down around the root of the nerve to the extra- 

 cranial portion. Preceding the stage of 69 hours, and particularly 

 before the formation of a fibrillated root, it is not possible to dis- 

 tinguish a definitive jugular ganglion from those cells which will 

 form the extra-cranial portion. The history of the jugular gan- 

 glion seems to be briefly as follows : First there is a loose mass of 

 cells extending from the brain ventrally to a point where the epi- 

 branchial placodes will be formed (fig. 75). This is followed by 

 a stage in which the visceral ganglion develops and the fibrillated 

 root is present, and in which the jugular ganglion surrounds the 

 root, sometimes thicker on one side, sometimes on the other, and 

 extends down over the root to the enlarged extra-cranial portion. 

 This is followed by a stage in which there is a definitive ganglionic 

 mass present, which owing to the development of the cartilaginous 

 skeleton can be designated as intra-cranial. This is connected 

 with the extra-cranial portion by a mass of spindle shaped cells 

 only, lying on and among the fibres of the root of the Xth nerve. 

 These spindle shaped cells I interpret as sheath cells, so that the 

 jugular ganglion is now quite distinct and can be easily differenti- 

 ated from the extra-cranial communis ganglion after 113 hours in 

 Ameiurus. 



There is little change in the appearance of this ganglion until 

 the stage of 81 hours, when the fibrous root of the Xth nerve is 



