344 F. L. LANDACRE 



into the primordium of the epibranchial placode, the histological 

 differences of the two structures and the fact that the one differ- 

 entiates after the other has lost its characteristic structure show 

 that the relation of the two placodes is more apparent than real. 

 Added to this we have the evidence to be shown later that there 

 is absolutely no relation between the postauditory placode and 

 the epibranchial placodes of the IXth and Xth nerves. 



The condition of the epibranchial placode in a somewhat older 

 embryo in which the placode is still in contact with the epidermis 

 is shown in figs. 25 to 30 (A. nebulosus, Stage VII). Fig. 25 

 is taken just posterior to the point of most intimate contact of the 

 hyoid gill pocket with the epidermis. In this series the contact 

 of the hyoid pocket with the epidermis, similar to that shown in 

 fig. 19, (Stage VI) occupies only one section, while in the series 

 from which fig. 19 was taken the contact is four sections in length. 

 The hyoid pocket after coming into intimate relation with the 

 epidermis gradually withdraws, and in the next stage following 

 that from which fig. 25 was taken no longer reaches the epidermis 

 at all. 



Fig. 25 is taken at the extreme anterior end of the placode and 

 the placode here differs from the epidermis anterior to it only in 

 being somewhat thicker and in having its nuclei irregularly ar- 

 ranged in more than one row. 



Fig. 26 is taken from the next section posterior to that from 

 which fig. 25 was taken and there is here a decided thickening 

 of the epidermis with numerous mitotic figures in various stages. 

 In the next section (fig. 27) the thickening projects mesially 

 as a well defined mass of cells and there are no less than twelve 

 cells in various stages of mitosis in the placodal region of the epi- 

 dermis. In the succeeding section (fig. 28) the proliferated mass 

 of cells, the anterior rJortion of the future epibranchial ganglion, 

 is larger and contains mitotic figures in addition to those in the 

 epidermis. The ganglion is still attached to the epidermis in all 

 these areas by a pedicle fully as thick as the ganglion itself. Two 

 sections posterior to this point (fig. 29) the ganglionic prolifera- 

 tion changes somewhat in appearance, being longer and its at- 

 tachment involving more of the epidermis ventral to it, so that 



