THE CRANIAL GANGLIA IN AMEIURI S oS, 



to the point of contact of the endodermal pocket of the hyoid 

 gill slit with the epidermis. This thickening is characterized 

 by the irregular arrangement of its nuclei and by the large num- 

 ber of mitotic figures. The thickening of the epidermis is fol- 

 lowed by an active proliferation of cells mesially, which come into 

 contact with the ventral portion of the lateral mass in this region. 

 The proliferated mass later becomes detached and after some hours 

 the geniculate ganglion, which is thus composed of a lateral mass 

 contingent and a placodal contingent, assumes definite form and 

 comes into intimate relation with the Gasserian ganglion. 



12. The epibranchial placode of the first true gill slit arises 

 in a similar manner, appearing first as a slight thickening lying- 

 dorsal and posterior to the first gill slit. The thickening is accom- 

 panied by active mitosis, proliferation of cells, and finally by com- 

 plete detachment, en masse, of the proliferated cells. The pro- 

 liferated epibranchial ganglion is in this case, however, apparently 

 a pure placodal ganglion, since no lateral mass cells could be 

 detected entering into its composition. Its dorsal extension comes 

 into contact with the remainder of the IXth before the ganglion is 

 completely detached from the epidermis however. 



13. The epibranchial ganglia of the second and third gill arches 

 have exactly similar modes of origin, but their dorsal extensions 

 soon come into contact with the lateral mass portion of the Xth. 

 While the second and third epibranchial ganglia are definite in 

 outline and mode of origin, their proximity to the Xth makes it 

 difficult to be sure that there may not be lateral mass cells in 

 their composition. 



14. The epibranchial ganglion of the fourth and fifth true gills, 

 owing to the fact that they are in the region of the large lateral 

 mass ganglion of the Xth, present a somewhat different history. 

 The fourth can be detected before the lateral mass of the Xth 

 comes into contact with it, and while its early stages resemble 

 those anterior to it, it does not become detached before the fusion 

 occurs between it and the lateral mass portion. In the case of 

 the fifth, the epibranchial ganglion cannot be detected in my series 

 before the fusion, so that while there is every reason for think- 

 ing that the fifth and sixth epibranchial placodes contribute cells 



