LATERAL CANAL SYSTEM OF SELACHIANS 45 



The nerve distribution to the .sensory cord is continuous 

 throughout its length. The cells of the cord have not taken on 

 a definite arrangement, although the different types of cells 

 characteristic of the adult appear to be differentiating. 



Twenty millimeter stage. In this specimen (fig. 39) is first 

 seen the rudiment of the dorsal series of pit organs. It is a 

 narrow, slightly thickened cord of cells arising as an off-shoot 

 from the suprabranchial ectoderm in a manner entu^ely com- 

 parable to the origin of the lateral and head cords. There is 

 a little ectodermal pocket at its growing end and it carries nerve 

 fibers from a branch of the lateralis ganglion (fig. 40). 



The lateralis and vagus ganglia (fig. 40) are somewhat more 

 advanced than in previous stages. The branch that innervates 

 the series of pit organs mentioned above is showai in figure 40. 

 It springs from about the middle of the lateralis ganglion and 

 extends obliquely upwards and backwards. 



As shown in sections the lateral nerve is still in close associa- 

 tion with the inner surface of the sensory cord (fig. 41). Farther 

 caudad the lateral nerve is separated into four bundles (fig. 42). 

 Transverse sections (figs. 42, 43) of the sensory thickening show 

 its relative size and degree of differentiation at different levels. 

 A distinct enlargement is evident at the distal end where the 

 thickening is composed of several layers of cells, many of which 

 show the mitotic figure. Anterior to the epidermal tunnel 

 (fig. 42) the cells of the sensory cord show considerable differ- 

 entiation and longitudinal invagination of the sensory epithe- 

 lium appears to have begun. 



Fig. 37 Surface view of an 18 mm. embryo. The lateral, supraorbital, and 

 infraorbital sensory cords have greatly extended in length. The branchial sense 

 organs of Beard are also quite prominent. From a sketch by Otto Swezey. 

 X 12. 



Fig. 38 Horizontal section of the growing end of the lateral sensory cord. 

 Squalus, 18 mm. See figure 43. 



Fig. 39 Surface view of a 20 mm. embryo. In this specimen the dorsal cord 

 is well defined and the rudiments of two supratemporal pit organs {Pt. 0.) have 

 appeared. Squalus. From a sketch by Otto Swezey. X 6. 



Fig. -40 Lateral dissection of the medulla of a stage similar to that shown in 

 figure 39. The rami which supply the supratemporal commissure and the dorsal 

 line of pit organs appear as small offshoots from the lateralis ganglion. X 13. 



