372 F. L. LANDACRE 



There is a well defined space, however, between the anterior end 

 of the lateralis IXth and the posterior end of the auditory gan- 

 glion. The backward growth of the vesicle carries the ganglionic 

 mass of the lateralis IX into the region occupied in some series by 

 the condensation of the lateral mass, so that it is impossible to 

 tell whether these lateral mass cells enter into the lateralis IXth 

 or not. If they do, the lateralis IXth resembles the auditory 

 rather than the lateralis Xth, which is derived solely from the 

 postauditory placode. 



Of the four acustico-lateralis ganglia in Ameiurus, the double 

 ganglion belonging to the Vllth is derived solely from the lateral 

 mass and shows its affinities with the general cutaneous Gasserian 

 ganglion most closely, not only in the source from which it comes 

 but in its mode of origin. 



The lateralis Xth, derived from the postauditory placode, is 

 decidedly unlike the lateralis Vllth, both in general appearance 

 and mode of origin. It is from the early stages a well defined 

 rod-like mass of cells whose long axis coincides with the long- 

 axis of the body, and is derived exclusively from the postauditory 

 placode. Intermediate between these two extremes lie the audi- 

 tory and lateralis IXth, which come largely if not exclusively from 

 the auditory vesicle, but may have lateral mass cells in their 

 composition. 



The lateralis Xth is the most highly specialized of the four, and 

 the reversal of the order of specialization, which usually proceeds 

 from anterior to posterior, is at first sight striking. The whole 

 acustico-lateralis system is, however, strictly a cranial specializa- 

 tion, the ganglia and nerves appearing first in the head and extend- 

 ing from there to the body, and having its center in the cranial 

 region. The change from the generalized lateralis ganglion of 

 the Vllth to the specialized lateralis ganglion of the Xth is in line 

 with the order of appearance of the lateral line organs, which 

 arise first in the cranial region, and later appear on the body. 

 While usually a highly specialized structure lies anterior to a less 

 specialized one, in the case of the acustico-lateralis ganglia the 

 structure having a specialized mode of origin lies posterior to the 

 one with a more generalized mode of origin. 



