56 SYDNEY EVANS JOHNSON 



growth of all parts of the lateral line system. The head canals 

 are closed at their proximal ends, and a short cord, sometimes 

 designated the temporal cord, lacks only a little of uniting the 

 lateral and the infraorbital canals. 



In the trunk region we can readily distinguish (as also in 

 earlier stages) a dorsal line, an accessory line and a lateral sen- 

 sory cord. 



The growing end of the dorsal cord, extending to the first 

 dorsal fin is still prominent. 



The gromng end of the dorsal cord is still prominent, although 

 intermediate parts of it have disappeared, leaving a series of 

 separated sensory thickenings which are still enclosed in little 

 epidermal pockets. This is shown in section in figure 59. 



That part of the lateral sensory cord in front of the epidermal 

 tunnel now appears as a continuous chain of minute 'swellings.' 

 These swellings are the little clusters into which the hair-cells 

 Tiave grouped themselves and which are characteristic of the 

 adult sensory epithelium. A longitudinal section of this part 

 of the cord (fig. 60) shows basal cells, columnar cells and sensory 

 cells. As shown in cross section (fig. 62) the sensory column is 

 sunken below the surface but is not as yet enclosed within a 

 canal. 



Immediately above the lateral cord is a series of separated 

 'swellings,' just a little larger than the cell clusters of the lateral 

 cord, and less numerous. These have been termed accessory 

 lateral line organs. Their appearance in longitudinal section 

 is represented in figure 61. The origin of these organs has not 

 been traced from their earliest appearance. There appears to 

 be little question, however, but that they arise from the superior 

 margin of the thickening which giyes rise to the organs of the 

 lateral canal. Evidence in favor of this view is seen in figure 63, 

 where the lateral and accessory organs are in close association, 

 both lying within the epidermal tunnel (also fig. 55). As the 

 epidermal fold recedes, the organs become uncovered and may 

 be readily seen in surface studies under low magnification. 



Thirty-six millimeter stage. In this embryo the course of the 

 canals is completely mapped out with the exception of the su- 



