ORIGIN OF THE LATERAL LINE PRIMORDIA 607 



end grows rapidly back along the side of the body and in Stage 

 XXIII is more than 124 sections long. 



In this same series there has appeared another primordium, 

 the second in position, and anterior to the third or between the 

 first and third primordia, as shown in figure 5, and near the an- 

 terior end of the lateralis X ganglion, whose lateral line organs 

 are supplied by the ramus supratemporalis X (figs. 5 and 33). 

 In Stage XXIV, where the nerve is sufficiently well developed to 

 trace it into the ectoderm in the vicinity of this third thickening, 

 the thickening lies with its anterior end over the second epibran- 

 chial thickening and extends from this point back beyond the 

 epibranchial thickening. 



In Stage XXVI, plotted by Landacre ('12), the lateral line 

 ganglion on X is shown as a single ganglion, quite long, from 

 the anterior end of which the ramus supratemporalis X arises. 

 This plot represents accurately the conditions at this stage. But 

 in earlier series XXIII and XXIV, there seems to be a second 

 small lateralis ganglion at the anterior end of lateralis X from 

 which the ramus supratemporalis X arises. The separation be- 

 tween the two is not absolutely sharp but in view of the findings 

 in the frog (Landacre and McClellan '13) and in the urodeles, 

 as shown in anunpublished paper by Kostir, it is extremely prob- 

 able that there are two lateral line ganglia on X which later, as 

 in the Anura and urodeles fuse into one common ganglion. 



In addition to the organs of the main lateral line innervated 

 by the ramus supratemporalis of X and those innervated by 

 the chief lateralis X ganglion, there are in series XXVII addi- 

 tional organs lying above the main lateral line ganglion inner- 

 vated by that ganglion. 



As to the relation of these three primordia to each other, there 

 is no evidence from our series, although they are not as close to- 

 gether as could be desired, that all three arise from a common 

 primordium. They appear to arise as three separate primor- 

 dia. It is practically certain that the first lateral lime primor- 

 dium supplied by the ramus supratemporalis IX is not connected 

 genetically with those lying posterior to it. Its extremely small 

 size in Stage XIX and evident increase in size in Stage XX, 



THE JOURNAL OP COMPARATIVE NEUROLOGY, VOL. 23, NO. 6 



