THE CRANIAL GANGLIA IN AMEIURUS 351 



At this stage in the formation of the ganglion there is nothing 

 to indicate its presence except the slightly denser massing of the 

 cells and the somewhat denser character of the inter-nuclear 

 material. There is one rather striking difference between the 

 Xth and preauditory ganglia. Anterior to the ear all the ganglia 

 trend down and forward from their points of attachment to the 

 brain where the fibrillated roots will appear. The Xth ganglion 

 derived from the lateral mass in the same manner trends down 

 and back from the point where its fibrillated root will appear, re- 

 versing the relations to the medulla. These figures are taken 

 from an embryo in which the condensations in the region of the 

 IXth and Xth ganglia are usually well marked. From this stage 

 up to the seventy-fifth hour in A. melas there is no well defined 

 ganglion even in the region of the Xth, and in several series I can 

 find no evidence of either ganglion. Between the 49-hour and the 

 56-hour embryo, A. melas, there is nothing that can be identi- 

 fied positively as a Xth ganglion, although one can be quite sure 

 of the region in which it ought to be located. In embryos of 61 

 to 65 hours, A. melas, there are slight condensations in the region 

 of the Xth, but they would hardly be detected if one did not read 

 back from older series. The ventral derivative of the lateral 

 mass (Y, figs. 41, 42) is present in the region of the Xth during 

 this time, however, and it is this region which furnishes the greater 

 portion of the visceral Xth. During the whole time under dis- 

 cussion the lateralis Xth ganglion is present and furnishes a good 

 landmark in the attempt to locate the early stages of the visceral 

 Xth. 



By the seventy-fifth hour (A. melas), the ventral portion of the 

 Xth ganglion derived from the lateral mass has assumed a definite 

 form and can be followed from this time on easily. It occupies 

 at this stage a ventro-mesial and mesial position with reference to 

 the lateralis Xth which is an elongated cylindrical mass of cells 

 with quite definite boundaries. The visceral Xth extends from 

 near the anterior boundary of the lateralis Xth to a point beyond 

 its middle portion and the motor root of the Xth extends along the 

 mesial side of the lateralis Xth running forward and upward to the 

 medulla. At this stage the lateral mass ganglion of the Xth is 



