48 SYDNEY EVANS JOHNSON 



In horizontal section the growing end of the lateral cord of 

 this stage appears practically the same as that shown in figure 

 36. 



Figure 46 is a cross section in front of the ectodermal pocket 

 near the pectoral fin. The nerve cord is well defined in this 

 situation. It lies close against the sensory cord and produces 

 a slight bulge on the surface. 



Twenty-two and one-half millimeter stage. In this stage 

 (fig. 47) it is important to note two little off-shoots of the thick- 

 ened ectoderm which pass dorsalwards posterior to the ductus 

 endolymphaticus. They take their origin from the supra- 

 branchial ectoderm and ectodermal folds are pushed up at their 

 distal ends. These small cords are substantially similar to 

 the other sensory thickenings. Like the dorsal cord, they 

 undergo subsequent partial degeneration or, at least, great 

 modification. In the adult each of the two small cords is rep- 

 resented by single isolated pit organ, while the dorsal cord is 

 represented by a series of separated pit or surface organs. 



The dorsal cord is still short, although it has thickened con- 

 sideral)ly and has a prominent fold of ectoderm at its distal 

 end. 



The supraorbital and infraorbital cords have not only become 

 greatly extended, but they have also begun to invaginate or 

 sink beneath the surface preparatory to the formation of closed 

 canals. 



The lateral cord extends as far caudally as the posterior 

 border of the first dorsal fin. The ectodermal tunnel forms a 

 long tube over its growing end, although its anterior lip has 

 also receded to a considerable extent. 



Fig. 46 Transverse section of lateral sensory cord at level of pectoral fin. 

 Note the slight bulging caused by the relatively large lateral nerve. Squalus, 

 21mm. X 592. 



Fig. 47 Surface view of a 22 mm. embryo. Squalus. P'rom a sketch by Otto 

 Swezey. X 625. 



Fig. 48 Dorso-lateral dissection of a 22| mm. specimen showing the nerves 

 concerned in the innervation of the various sensory cords. Squalus. X 13. 



