ORIGIN OF THE LATERAL LINE PRIMORDIA 603 



second true gill, this thickening becomes one with the thickening 

 in the lateral angle of the body, so that it is not possible to deter- 

 mine exactly how much of it lies behind the area of contact. 



Stage XVIII. Stage XVIII is characterized by a marked de- 

 gree of separation of the thickening at the contact of the second 

 true gill into two parts at its anterior end (fig. 3). Figure 18 

 shows the detail in the middle of the thickening extending behind 

 the first gill contact. The portion of the second thickening lying 

 above the gill slit represents partly the area of contact but mostly 

 a posterior extension behind the area of contact of the second true 

 gill. 



The part lying below the split represents the thickening at the 

 angle of the body. This will be chiefly contact of the third gill 

 pocket in the next series. The two thickenings are continuous, 

 as indicated in figure 3, throughout the posterior three fourths. 

 Figure 31 shows the appearance of the two thickenings just 

 behind the area of contact of the second true gill. The whole 

 ventral half of the second thickening lies at the angle where the 

 ventral lateral body wall becomes horizontal as it passes out 

 over the yolk. The posterior end of this combined dorsal and ven- 

 tral thickening extends backward into the region of contact of 

 the third true gill. 



Stage XIX. Stage XIX (fig. 4) shows striking changes in 

 the further differentiation of gill thickenings, as well as in the 

 appearance of the first trace of the postauditory lateral line 

 primordia. 



In the region of the second true gill (fig. 4) the thickening 

 which in the preceding stage was confluent at its posterior end 

 with a more ventral thickening is now quite distinct and ends 

 free behind. The second thickening plotted in figure 4 is mainly 

 a posterior extension behind the area of contact, while the third 

 thickening represents mainly the area of contact of the third 

 true gill and at its posterior end drops to a lower level than at 

 its anterior end, and occupies again the angle at the side of the 

 body. It also extends well back into the area of contact of the 

 fourth true gill. 



