THE CRANIAL GANGLIA IN AMEIURUS 343 



Tracing the placode back in a number of series leaves no doubt 

 that this is the primordium of the placode. The manner in which 

 this thickening of the epidermis increases in size and finally be- 

 comes detached, forming a portion of the geniculate ganglion, 

 is quite easy to follow. Of the fate of the small mass of cells 

 (x) shown in figs. 21 and 22, I am not certain. They may enter 

 into the ganglion, but it is more probable that they become con- 

 verted into mesectoderm. They lie a little anterior to the point 

 of origin of the ganglion, and there is in some of my later series 

 a small mass of cells anterior to the point of contact of the gill 

 pocket with the epidermis, which resembles these, but their 

 origin and fate I cannot determine definitely. . If the cells shown 

 enter into the ganglion, it is by attaching themselves later to the 

 main ganglionic mass derived from the placode. Of this, however, 

 I have no evidence. 



Figs. 22 and 23 show a typical appearance of the early stage of 

 any epibranchial placode and resemble closely the first stages of 

 the placodes of the IXth and Xth epibranchial ganglia. The 

 nuclei are several layers deep and quite irregular in arrangement 

 and mitotic figures are beginning to be quite numerous. Fig. 

 24 is taken four sections posterior to fig. 23 and shows the transi- 

 tion of the placode into ordinary epidermis. Here, however, 

 mitotic figures are still rather numerous but two sections posterior 

 to this the epidermis is unmodified. 



A comparison of the embryo from which figs. 19 to 24 (A. neb- 

 ulosus, Stage VI) were taken with the one from which figs. 17 and 

 and 18 (Stage V) were taken leaves little doubt that, while the 

 epibranchial placode appears practically in the place where the 

 preauditory placode disappeared, there is no direct relation be- 

 tween the two histologically. All of the preauditory placode pos- 

 terior to the hyoid gill pocket is converted into mesectoderm and, 

 while there is no evidence that the placode at the point of contact 

 is ever converted into mesectoderm beyond the presence of the 

 small mass of cells mentioned above, the fact that a contact with 

 endoderm is formed would obscure the conditions here somewhat. 

 While it is impossible to say that none of the cells that were once 

 a part of the preauditory placode or their direct descendants enter 



