THE CRANIAL GANGLIA IN AMEIURUS 337 



end of the vesicle lose their characteristic regular outline, while 

 throughout the remainder of the vesicle its walls are still charac- 

 terized by their clean cut boundary and by the presence of a 

 single row of distally located nuclei. At the anterior end, however, 

 the ventral wall becomes indistinguishable and its nuclei become 

 several layers deep and there is no perceptible boundary between 

 the vesicle wall and the forming VHIth ganglion. 



Fig. 13, Stage V, taken from the same embryo as figs. 10 and 11, 

 lies four sections back of the anterior end of the auditory vesicle. 

 The mingling of the vesicle and ganglionic cells and the numerous 

 mitotic figures in this location indicate without doubt that the 

 vesicle contributes cells to the ganglion. If it were not for the 

 fact that the vesicle is constantly in contact with the ganglionic 

 mass derived from the lateral mass, the lateralis Vllth ganglion, 

 and seems to grow forward into this mass on its dorsal and 

 mesial wall, one would be inclined to think that the whole audi- 

 tory ganglion came from the vesicle. The conditions here are 

 identical with those at the posterior end of the vesicle where the 

 lateralis IXth ganglion is formed. That ganglion is proliferated 

 from the wall of the vesicle after the vesicle is formed and can be 

 followed from its first appearance until the vesicle ceases to con- 

 tribute cells to it. The conditions at the posterior end of the 

 vesicle are not complicated to the same extent by the presence of 

 any contiguous ganglionic mass and one can be much more cer- 

 tain that the whole ganglion comes from the vesicle. Since, how- 

 ever, at the anterior end of the vesicle the cells derived from the 

 vesicle are in contact with those derived from the lateral mass and 

 there is no definite division into an Vlllth ganglion and two later- 

 alis Vllth ganglia for some time after this, it is impossible to say 

 positively that the whole of the Vlllth is derived from the vesicle. 

 This mode of derivation is indicated by the conditions, however, 

 and is further strengthened by the positions of the lines of cleav- 

 age separating the Vlllth from the lateralis Vllth which appear 

 later, as well as by the manner in which the Vlllth ganglion 

 adheres to the vesicle for a long time (figs. 35 to 39 and fig. 83). 



I shall defer a discussion of the various ways in which the acus- 

 tico-lateralis system of ganglia arises until after the description 



