54 SYDNEY EVANS JOHNSON 



has undergone considerable cellular differentiation so that the 

 main featui-es of the histology of the sense organs are discern- 

 ible. There are columnar cells, basal cells and sensory cells. 

 The outer wall of the ectodermal tunnel is very thin and shows 

 e\ddence of degeneration. 



Above the lateral sense organ proper is seen in this section an 

 accessory organ which appears to have budded off from the 

 lateral sensory cord and is still within the ectodermal fold (fig. 

 55). A section anterior to the ectodermal tunnel (fig. 56) 

 exhibits the accessory and the canal organs more widely sepa- 

 rated and the lateral nerve considerably removed from the inner 

 surface of the ectoderm. 



Accessory sense organs of this kind exist as a longitudinal 

 series running parallel to the lateral sense organs. They are 

 shown in figure 52 just dorsal to the sensory cord. In this 

 specimen they are exposed from the first branchial arch to the 

 dorsal fin and then disappear under the ectodermal fold. 



One of the two small sensory cords lying near the yolk-stalk 

 (fig. 53, also fig. 3) is shown in longitudinal section in figure 57. 

 The general resemblance of this section to one of the growing 

 ends of the lateral cord is striking. It has an ectodermal fold 

 and the sensory cord is well provided with nerve fibers. These 

 thickenings are similar to the other sensory cords except that 

 they are more transitory and I was not able to trace their origin 

 from the thickened suprabranchial ectoderm. Of these cords 

 only a few isolated pit or surface organs remain in the adult. 



Twenty-nine milhmeter stage. In this embryo (fig. 58) the 

 surface changes are not great and consist in the progressive 



Fig. 55 Transverse section of the lateral sensory cord posterior to the first 

 dorsal fin. Note that a lateral line organ and an accessory organ are both cov- 

 ered by the superficial epidermal fold. Squalus, 27 mm. X 492. 



Fig. 56 Transverse section of the lateral sensory cord farther cephalad 

 (anterior to the epidermal tunnel) than shown in figure 55. Here the lateral 

 nerve lies at a deeper level, and the accessory organ is farther removed from the 

 lateral line organ. Squalus, 27 mm. X 370. 



Fig. 57 Longitudinal vertical section of one of the pit organ rudiments indi- 

 cated near the yolk stalk in figure 53 (Pt.O.). Squalus, 27 mm. X 492. 



Fig. 58 Surface view of a 28 mm. embryo. Squalus. X 3. 



