THE CRANIAL GANGLIA IN AMEIURUS 373 



THE ORIGIN OF THIRD AND FOURTH EPIBRANCHIAL PLACODES 



The history of the first placode entering into the composition 

 of the Xth ganglion, the third epibranchial, resembles very closely 

 that of the epibranchial placode of the IXth ganglion, except that 

 it forms its attachment to the brain in conjunction with the re- 

 maining roots of the Xth, by extending posteriorly until it joins 

 the roots of the fourth placode. It can be detected first in my 

 series (fig. 64) in a 69-hour embryo, at which stage it resembles 

 very closely the early stage of the placode of the IXth nerve. In 

 the next series (75 hours) the placode has proliferated quite a mass 

 of cells mesially, but is not free from the epidermis at any point 

 in its length, although three sections from its anterior end there 

 is an evident constriction between the ganglionic mass and the 

 epidermis. 



In an 81-hour embryo the ganglion is not in contact with the 

 epidermis at its posterior end, and has acquired a connection with 

 the mass of cells lying over the site of the fourth and fifth epibran- 

 chial placodes of the Xth (fig. 79). In an 86-hour embryo the 

 ganglion is still attached to the epidermis, but is somewhat larger 

 and its root somewhat more evident. In a 93-hour embryo i^figs. 

 80 and 65) the ganglion is still attached at its anterior end. In fig. 

 66, 93 hours, taken from a section following 65, the ganglion is 

 attached by a very narrow neck of cells and throughout the re- 

 mainder o f its length is entirely free (figs. 67, 68, 69, 70) extending 

 posteriorly into a narrow neck of cells which connects it with the 

 last division of the Xth nerve. The root of this nerve, as mentioned 

 above, lies lateral to the cardinal vein, as do all the other placodal 

 rootsof theXth. The appearance of the epidermis near the placode 

 resembles closely that in the region of the IXth. Anterior, dorsal 

 and posterior to the placode the epidermis is usually one or two 

 cells thick and quite ragged, presenting the appearance of pro- 

 liferating cells into the mesectoderm. The thickening of the plac- 

 ode extends ventrally in the epidermis and, as in the case of the 

 IXth, this region presents the appearance of furnishing most of 

 the cells that move into the ganglion. Mitotic figures are occa- 

 sional both in the epidermis and the placode. 



