606 F. L. LANDACRE AND A. C. CONGER 



settle this interesting point. Stages XXI and XXII are unfor- 

 tunately the only stages in the series up to Stage XXX that are 

 more than seven hours apart and even this interval probably 

 would have been too long to settle it. 



The most posterior primordium of the postauditory lateral 

 line, the third in position, lies posterior and, at its anterior end 

 at least, dorsal to the third gill thickening (fig. 5). Its anterior 

 end begins at the transverse level of the posterior end of the 

 third gill thickening, and from this point it extends back entirely 

 beyond and dorsal to the fourth gill thickening. Behind the 

 fourth gill thickening it drops to a lower level but not as low as 

 the angular thickening on the side of the body. The angular 

 thickening in this stage does not extend posterior to the posterior 

 end of the future fifth gill thickening, so that the third lateral 

 line primordium extends posterior to the gill region. 



The relation of this lateral line primordium to the lateralis X 

 ganglion is interesting. The lateral line ganglion of the X oc- 

 cupies 200 Ai. It is largest in its middle region and at its posterior 

 end comes quite close to the ectoderm (fig. 34). The lateral 

 line primordium is 300 n in length and its anterior end overlaps 

 the first five sections (50 m) of the lateralis X ganglion where it 

 comes quite close to the ectoderm. In fact, the two structures 

 are almost in contact, suggesting strongly, as was indicated in 

 discussing the more anterior lateral line primordium, that the 

 ganglion is derived by splitting off from the lateral line placode. 

 From the posterior end of the ganglion the lateralis X nerve runs 

 back only 50 pL beyond the posterior end of the ganglion, leaving 

 200 M of the lateral line primordium with no accompanying nerve, 

 a fact which furnishes rather striking evidence that the lateral 

 line nerve is not split off from the ectoderm but grows out from 

 the ganglion, as other nerves do. 



Histologically the posterior lateral line primordium shows less 

 resemblance in these earlier stages to the lateral line organs than 

 do those in the hyoid region. This resemblance occurs a little 

 later and its absence at this stage may possibly be accounted for. 

 by the sudden appearance and rapid growth of the structure. 

 The later history of this primordium as as follows: The posterior 



