50 SYDNEY EVANS JOHNSON 



Figure 48 represents a lateral dissection of a 22.5 mm. embryo 

 showing the ganglia and the nerves associated with the sensory 

 cords. Note again in this stage the branch to the dorsal pit 

 organs and the one to pit organs of the supratemporal region. 

 Anteriorly the lateral nerve has now come to lie slightly below 

 the integument, but a little farther back it is still in close con- 

 tact with the ectoderm (fig. 49). 



Twenty-five millimeter stage. In this embryo (fig. 50) 

 marked growth has occurred in the supraorbital cord, its in- 

 ferior limb now bending backward in front of the eye. The 

 infraorbital is considerably extended also, and passes to the ven- 

 tral side of the head. The infraorbital and supraorbital both 

 present slight grooves, and at their proximal ends the lips of 

 the grooves have begun to approach each other in short sec- 

 tions or segments. The spaces between the sections with ap- 

 proximated lips become smaller in advanced stages and remain 

 as the pores of the tubules in the adult. 



The dorsal cord has pushed back a little farther and has a 

 long epidermal tunnel at its growing end. 



The lateral cord now extends to the posterior border of the 

 anal fin and the ectodermal tunnel extends forward to the first 

 dorsal fin. It is to be kept in mind that the ectodermal tunnel 

 is a temporary structure and is not to be confused with the canal 

 which is a later formation. 



A perceptible groove has appeared on the surface of the cord 

 anterior to the opening of the ectodermal tunnel and the mouth 

 of the tunnel has receded to the first dorsal fin. 



A lateral dissection of a specimen of the same length is shown 

 in figure 51. In front are shown the ganglia from which issue 

 the nerves that supply the supraorbital and the infraorbital 

 lines. Posteriorly is exposed the long lateralis nerve. Two 

 branches are given off from the body of the ganglion. The 

 anterior one goes to the supratemporal commissure and the 

 posterior to the dorsal series of pit organs. 



Twenty-seven millimeter stage. Surface views of this embryo 

 are represented by figures 52, 53. The anterior canals have 

 extended to the ventral surface of the head. The supraorbital 



