362 F. L. LAND ACRE 



tory vesicle. If these organs arise as differentiations of sensory 

 ridges which grow out from the anterior and posterior extension 

 of the auditor}'' vesicle, they cannot be strictly homologous with 

 the sensory areas of the auditory vesicle, since they are derived 

 from it, unless we consider the ear as precociously developed on 

 this ridge. If on the other hand the lateral line organs arose 

 primitively as single organs, the sensory areas of the auditory 

 vesicle and lateral line organs can be considered as homologous 

 and the conditions in Serranus, Salmo, Batrachus, and Amia can 

 be explained as modifications of this primitive method. 



THE PLACODAL GANGLION OF THE IXth NERVE 



The visceral ganglion of the IXth nerve stands in a rather 

 unique position since it has no recognizable constituent derived 

 from the lateral mass, and is therefore a pure placodal ganglion ; 

 otherwise it presents many points in common with the placodal 

 ganglion of the Vllth and those of the Xth to be described later. 

 It begins as a thickening of the epidermis on a level with the 

 dorsal limit of the first true gill slit, and extends posteriorly from 

 this point. The thickening is characterized by the irregular 

 arrangement of its nuclei, and by the number of mitotic figures. 

 The stage of thickening is followed by a process of proliferating 

 cells mesially and posteriorly, and this group of proliferated cells 

 is finally detached en masse and acquires a connection with the 

 more dorsally situated lateralis ganglion. Fig. 53 from a 562-hour 

 embryo (A. melas) is taken just posterior to the point of contact 

 of the visceral pocket with the epidermis. The placode is recog- 

 nizable at this stage as a simple thickening of the epidermis ex- 

 tending from the level of the dorsal border of the future gill slit 

 down to the level of the branchial artery and longitudinally occu- 

 pying three or four sections. The epidermis is several cells thick 

 and usually, especially during later stages, shows many mitotic 

 figures. The epidermis is nearly always slightly indented at the 

 point where the placode appears, but whether this is to be associ- 

 ated with the placode or with the gill slit is not clear. The pla- 

 code differs structurally from the epidermis adjoining it in the 



