GILLS IN THE YOUNG NECTURUS. 1/3 



rical patterns. While the lateral buds are elongating the tip of 

 the gill bar becomes drawn out into a median filament, as in Ser. 

 III., Fig. 21. Sometimes the tip of the gill bar forms one of the 

 first pair of filaments, in which case a median bud forms the me- 

 dian or apical filament. 



A second pair of filaments forms some 48 to 60 hours later. 

 In most cases these are bilaterally symmetrical, as indicated in 

 Fig. 21. In two out of the eight series the buds did not arise at 

 opposite points, and in these cases more or less irregularity oc- 

 curred in the succeeding pairs. 



The third pair of filaments shows considerable variation both 

 in time and place of origin. In five cases they appeared about 

 48 hours alter the second pair. In the remaining three cases they 

 were not formed until the end of the third day. The filaments 

 of this pair usually arise in such positions that they are bilaterally 

 symmetrical, as shown in Ser. III., Fig. 26. In two series con- 

 ditions were observed something like that represented in Ser. II., 

 Fig. 26, where only one of the third pair arose in the usual posi- 

 tion. In place of the other there arose an intercalated bud on 

 the opposite side. 



The fourth pair of filaments may develop as bilaterally sym- 

 metrical structures, as in Ser. I., Fig. 30, or may be represented 

 by a single intercalated filament, as in Ser. III., Fig. 30, or again 

 the filaments may show such great irregularity that it is impos- 

 sible to tell what filaments are to be considered as belonging to 

 the fourth pair, as in Ser. II., Fig. 30. 



In the later growth of the gill the filaments not only continue 

 to form in pairs at the base of the gill bar but also to arise irregu- 

 larly from intercalated buds. In addition the filaments send off 

 lateral branches which form secondary filaments. In the older 

 larvae (3040 mm.) the patterns become more and more 

 irregular. 



As a general statement one might say that in the earliest fila- 

 ments much regularity prevails, but in the formation of the later 

 filaments the regularity decreases. 



When the gills had reached the stages shown in Fig. 30 they 

 were cut off with spring scissors as near the head as possible and 

 as they regenerated their changes were carefully followed and 



