THE NERVOUS SYSTEM OF AMPHIBIA 187 



In the embryo of the non-motile stage no lateral line primordia 

 occur caudad of the third myotome. Lying over the second 

 myotome laterally, and extending partially over the first and 

 third, is a broad primordium (fig. 56, Po. LL. 2), which lies im- 

 mediately dorsad of an ectodermal thickening allied with the 

 visceral system. In the embryo of the early-flexure stage (fig. 57) 

 there are two such primordia, one {Po. LL) situated over the 

 first myotome, and a long primordium {LL) extending over the 

 second, third, fourth and fifth myotomes. The latter is constricted 

 in its rostral portion, with apparently the tendency to separate 

 into two primordia. In the embryo of the coiled-reaction stage 

 the smaller primordium of the last stage seems to have coalesced 

 with another which lay near the caudal border of the auditory 

 vesicle, to form a single large primordium which extends from 

 the auditory vesicle to near the second myotome. Farther caudad 

 there are two primordia instead of one, as in the younger stage. 

 The more rostral of the two (fig. 58) is short and lies over the 

 third and fourth myotomes. The other extends through the level 

 of the seventh, eighth, ninth and tenth myotomes. 



As compared with the last embryo the oldest of the series (fig. 

 59) shows a considerable differentiation of the system of primor- 

 dia. There is one large primordium over the first myotome, 

 in addition to the one slightly cephalad and extending over the 

 auditory vesicle. Ventrad of the second myoseptum is a small 

 primordium {Inf. LL) which is situated just behind the external 

 gills. It presumably represents the inferior line. In the position 

 of the lateral line proper there are four distinct, short primordia 

 scattered along over the eighth to twelfth myotomes, and a long 

 one extending continuously through the thirteenth to seventeenth 

 myotomes. In this embryo a dorsal group of primordia also 

 appears, over the fourth, fifth, sixth and seventh myotomes, 

 near the level of their dorsal border. 



