THE CRANIAL GANGLIA IN AMEIURUS 375 



to 68 and shows the complete detachment of the posterior end 

 of the ganglion from the epidermis. Fig. 70 shows the appearance 

 of this ganglion seven sections posterior to the section from which 

 fig. 69 was drawn, and just anterior to its union with the anterior 

 portion of the lateral mass ganglion over the fourth gill slit. 

 Fig. 71 shows the point of union of this ganglion with the anterior 

 end of this lateral mass ganglion. The only difference between 

 this placode and the third epibranchial lies in the thickness of 

 its posterior portion, that part corresponding to the root of the 

 third epibranchial ganglion. It lies much nearer its destination 

 and apparently does not become so attenuated in reaching it. The 

 later history of this ganglionic mass (figs. 81 and 82, Ep. G. IV) 

 shows that it fuses much more closely with the ganglionic mass 

 lying posterior to it than does the third, and may contain lateral 

 mass cells, although I cannot be sure that they enter into its com- 

 position. It becomes completely detached from the epidermis in 

 an embryo of 105 hours; while both the third and fourth epibran- 

 chial ganglia can be recognized in my oldest series, the fourth 

 fuses much more closely with the remainder of the Xth than does 

 the third. 



THE ORIGIN OF THE FIFTH AND SIXTH EPIBRANCHIAL PLACODES 



In the case of the fifth and sixth epibranchial placodes condi- 

 tions are quite different on account of the fact that the lateral 

 mass enters so prominently into the composition of this portion 

 of the Xth. The relations are somewhat confused here on account 

 of the enormous size of the lateral mass at the time the fifth and 

 sixth placodes appear. The earliest trace of a placode I have been 

 able to find for the fourth gill slit is represented in fig. 70 (93 hours) . 

 This placode has the appearance that all the other placodes present 

 at the time they begin to proliferate cells mesially. The appear- 

 ance usually presented in my preparations is shown in figs. 72 

 and 73. The large lateral mass ganglion comes into contact with 

 the epidermis and remains in contact about the length of time the 

 other placodes retain their connection with the epidermis. Except 

 for the condition shown in fig. 70 it would not be possible to as- 



