THE CRANIAL GANGLIA IN AMEIURUS 



377 



unable to determine to what extent cells move from the placode 

 into the ganglion, since there seems to be no noticeable distinction 

 between placodal cells and lateral mass cells in any of my embryos. 

 The same reasoning applies here, however, that was used in the 

 case of the fifth epibranchial placode. The location, time of ap- 

 pearance, and manner of thickening of the epidermis resemble 

 the third and fourth placodes in their early stages when they are 

 undoubtedly contributing cells to these ganglia. Its more tran- 

 sient character is in keeping with the reduction which has occurred 

 in the fifth epibranchial ganglion as compared with the fourth, 

 but seems more marked. It indicates a reduction of the placodes 

 from posterior to anterior, and is to be associated with the reduc- 

 tion in the number of gills in the bony fishes as compared with cy- 

 clostomes and elasmobranchs. 



The time at which the contact occurs and the length of the at- 

 tachment of the IXth and the four epibranchial ganglia of the 

 Xth is shown in the following table : 



TABLE IV 



Showing the time at which the epibranchial placodes of the ninth and tenth ganglia 

 appear, the time at which they become detached, and the length of time of attach- 

 ment. 



This table shows that the fifth and sixth epibranchial placodes 

 appear in serial order and become detached from the epidermis in 

 the same order. They differ principally in having a shorter total 

 time of attachment. There seems to be no reason for supposing 

 that conditions are different here other than in the reduction of 

 the placodes and in the presence of a well defined lateral mass 

 ganglion which fuses with the placode. 



THE JOURNAL OF COMPARATIVE NEUROLOGY AND PSYCHOLOGT, VOL. 20, NO. 4. 



