LATERAL CANAL SYSTEM OF SELACHIANS 39 



eye. These cords represent the rudiments of the supraorbital 

 and the infraorbital sensory canals. Nerve fibers from two 

 ganglionic enlargements of the seventh are distributed among 

 the cells of the inner surface of the growing cords. The superior 

 bundle of fibers is the rudiment of the ramus ophthalmicus 

 superficialis VII and the inferior bundle the rudiment of the 

 ramus buccalis VII. The cord of the lateral canal is obviously 

 longer than in the earlier stages. 



Fifteen millimeter stage. Surface observations of this embryo 

 (fig. 33) show considerable growth of the sensory cords. The 

 supraorbital and the infraorbital are longer and better defined. 

 The nerve bundle of the infraorbital cord has begun at its prox- 

 imal end to sink beneath the inner surface of the cord. 



The lateral cord has increased in length and also in thickness. 

 A new feature has made its appearance at the distal end of the 

 cord in the form of an ectodermal fold (fig. 33, Ep. Po.) which 

 at first is a shallow pocket, and in later stages becomes a tunnel. 

 The condition produced is as if the distal end of the cord in 

 growing backwards had pushed under the common ectoderm 

 lying in its path, causing the latter to bulge into a crescent- 

 shaped fold. A longitudinal section of this fold is shown in 

 figure 36. 



A dissection of the region back of the ganglion of the ninth 

 nerve is shown in figure 34. The lateralis ganglion and its 

 roots are now easily distinguishable from the ganglia and roots 

 of the vagus. The vagus roots enter the brain wall in a contin- 

 uous broad sheet and the inferior margin is separated into four 

 distinct ganglia. The lateralis ganglion crosses the vagus- 

 complex running obliquely above the four ganglionic enlarge- 

 ments. It is greatly elongated, its distal end reaching posterior 

 to the fourth vagus ganglion. 



Longitudinal sections of this stage show the nature of the 

 growth of the lateralis nerve and the relation of its fibers to the 

 cells of the sensory cord. Figure 35 represents part of a sec- 

 tion taken lengthwise through the lateralis ganglion. The gan- 

 glion {Lat. Gn.) is composed chiefly of bipolar cells the central 

 fibers of which pass into the brain (fig. 28). The peripheral 



