374 F. L. LANDACRE 



There seems to be no question that here, just as in the case of 

 the second placode, we have first a thickening of the epidermis 

 above and behind the second true gill slit, followed by a prolifera- 

 tion of cells dorso-mesially to form the body and root of the gan- 

 glion, and later the complete detachment of the proliferated mass 

 from the epidermis. The attachment disappears last at the ex- 

 treme anterior end in an embryo of 99 hours. The dorsal exten- 

 sion of this placodal ganglion resembles closely the dorsal exten- 

 sion of the placodal ganglion of the IXth nerve; but the fact that 

 it grows back parallel to the dorsal extension of the fourth epi- 

 branchial ganglion, and that both soon come into contact with 

 the lateral mass ganglion of the Xth whose boundaries are at 

 this stage not well defined and which is surrounded by mesecto- 

 derm make it more difficult than in the case of the IXth to deter- 

 mine whether there are lateral mass cells in its composition. Gen- 

 eral visceral fibres, if present in this ganglion, might be traceable 

 either to lateral mass cells incorporated into the ganglion, or they 

 might easily grow into it from the large lateral mass ganglion of 

 the tenth, situated over the fourth and fifth gills. The conditions 

 are by no means as favorable for drawing a definite conclusion 

 as to the composition of the ganglion as in the case of the epi- 

 branchial ganglion of the IXth. 



The development of the fourth epibranchial placode which 

 gives rise to the fourth epibranchial ganglion resembles closely 

 that of the second and third. In an embryo of 81 hours the pos- 

 terior extension of the placodal ganglion has come into contact 

 with the lateral mass ganglion situated just over the fourth gill 

 slit at a point just ventral to that at which the root of the second 

 epibranchial ganglion joins the same mass (fig. 79) . 



I have been unable to detect the placode in a 69-hour embryo. 

 My 75-hour embryo is defective at this point. In the 81-hour 

 embryo the ganglion is in contact with the epidermis throughout 

 about half its length. The conditions are so similar here to those 

 of the IXth, and first placode of the Xth, that I am sure that the 

 method of appearance is the same. In fig. 68 (93 hours) is shown 

 the appearance of the placode at the anterior portion of its attach- 

 ment to the epidermis. Fig. 69 is taken two sections posterior 



