52 SYDNEY EVANS JOHNSON 



has lengthened and passes backwards between the eye and the 

 nostril, while the infraorbitals draw close to each other in front 

 of the buccal orifice and then bend forward a short distance 

 between the nostrils (fig. 53). The grooves of the head thick- 

 enings are now quite deep and at their proximal ends the lips 

 of the canals have begun to fuse or coalesce. 



As seen on the ventral surface the hyomandibular sensory 

 cord bifurcates at the angle of the mouth (fig. 53), one arm 

 passing forward and the other backward. 



The anlage of the supratemporal commissure has made its 

 appearance as a short ectodermal cord passing dorsalwards 

 immediately posterior to the opening of the ductus endolym- 

 phaticus (fig. 52). Two little rounded thickenings in front of 

 the ductus endolymphaticus represent the remains of the two 

 short thickenings shown in figure 47, and mentioned in the de- 

 scription of the 22 mm. embryo. 



The lateral cord has extended posteriorly as far as the second 

 dorsal fin, and the dorsal cord almost as far as the first dorsal. 

 A new cord has arisen between the anterior end of the lateral 

 cord and the point of origin of the supraorbital and infraorbital 

 canals, with which it unites some time later. 



A dissection of the lateralis ganglion and the adjacent ter- 

 ritory is shown in figure 54. Arising from the body of the gan- 

 glion are the branches that supply respectively the supratem- 

 poral commissure and the dorsal series of pit organs. The 

 lateralis nerve shows an interesting condition. It is divided into 

 two strands. From the upper strand are given off ramuli of 

 nerves that go to the accessory sense organs and from the lower 

 strand similar ramuli whose fibers supply the canal sense organs. 

 Dissections of other specimens make it clear that this division 

 into two strands is not always present. It was observed in 

 another specimen 29 mm. long, but in most of the specimens 

 dissected these strands were combined into a single nerve. 



A transverse section of the cord near the anterior end of the 

 epidermal tunnel (fig. 55) shows that the fibers of the lateralis 

 nerve have become gathered into a definite trunk which still 

 lies immediately under the sensory cord. The sensory cord 



