594 F. L. LANDACRE AND A. C. CONGER 



At a time, then, when the preauditory placode has shortened 

 so that 70 At intervene between its anterior limit and the posterior 

 limit of the epibranchial placode, the first primordium of the 

 sensory lines appears (fig. 2, 94 hours). Its cells do not ex- 

 hibit a pronounced radial arrangement in this stage, but the 

 cells are elongated, and the majority of the cell nuclei are found 

 in the mesial ends of the cells. 



The position occupied by the primordium of the sensory lines 

 in the 94-hour stage is dorsal and anterior to the position occu- 

 pied by the preauditory placode in the preceding stage. In 

 stages prior to the 94-hour stage (fig. 1), the preauditory placode 

 showed a sharp downward flexure at the point where it joined 

 the ectodermal thickening which extended caudad from the region 

 where the endoderm of the hyoid gill pocket approached the 

 epidermis. 



Examination of a number of individuals at the 94-hour stage, 

 exhibiting variations in the degree of development, fails to show 

 any evidence for a genetic relation between the preauditory plac- 

 ode and the primordium of the sensory lines. That there is no 

 such relation seems all the more probable, since the anterior end 

 of the placode at no stage occupied the region marked later 

 by the appearance of the primordium of the sensory lines, but 

 extended forward with a downward flexure in the region just 

 posterior to the epibranchial placode. This flexure is shown in 

 figure 1. 



The 100-hour stage (fig. 3) shows a further development of the 

 primordium of the sensory fines, in that the. radial arrangement 

 of the cells is more pronounced, so that the structure is well 

 differentiated by histological characters from the ectodermal hy- 

 oid thickening which lies at a sfightly lower level. The sensory 

 line primordium is 140 ^t in length in the 100-hour stage, and lies 

 parallel with the roof of the pharynx. 



The 106-hour stage (figs 4 and 21) exhibits little change over 

 the preceding stage as regards the development of the primor- 

 dium of the supra-orbital sensory lines. In this stage, however, 

 the first trace of the primordium of the mandibular line is found. 

 It lies ventral to the middle portion of the hyoid ectodermic 

 thickening and completely detached from it. In this case there 



