590 F. L. LANDACEE AND A. C. CONGER 



b. Ectodermal gill thickening 



The most conspicuous structure in the region between the audi- 

 tory vesicle and the optic vesicle in early embryos is the ecto- 

 dermal thickening where the endoderm of the hyoid gill pocket 

 approaches the ectoderm. The significance of this structure is 

 uncertain, as has been pointed out (Landacre '12), but its extreme 

 size in this type and the fact that it occupies approximately the 

 same level dorso-ventrally in which the sensory lines later ap- 

 pear makes it of importance in this discission. As the endoderm 

 of the hyoid gill pocket pushes out and approaches the epidermis, 

 a thickening appears at, anterior to, and posterior to the point of 

 contact (fig. 1). This ectodermal thickening lies at a slightly lower 

 level than that occupied by the preauditory placode. In stages 

 preceding the 88-hour stage, the ectodermal thickening extends 

 caudad from a point 20 n anterior to the contact of endoderm 

 and ectoderm, from which point it curves upward and joins the 

 anterior end of the preauditory placode. The ectodermal thick- 

 ening is easily distinguished from the preauditory placode, since 

 it is not marked by the peculiar cell arrangement so character- 

 istic of the placode. The cells of the thickening are small in 

 size, irregular in outline, and have deep-staining qualities. The 

 relation of the ectodermal thickening to the preauditory plac- 

 ode at the 88-hour stage is shown in figure 1. In this stage 

 the actual contact is 160 /^ in length and the thickening is of less 

 extent but more pronounced appearance than in earlier stages. 



The changes in the thickening from the 88-hour stage (fig. 1) 

 to the 120^ hour stage (fig. 5) . in which well developed sensory 

 lines can be recognized, may be followed by a comparison of 

 figures 1, 2, 3, 4 and 5. It will be seen that the ectodermal 

 thickening continues to decrease in length. The actual contact 

 of the endoderm and ectoderm shortens at the same time. In 

 the 94-hour stage (fig. 2) the contact is 140 ^ in length, and ends 

 posteriorly just ventral to the reg on marked by the appearance 

 of the primordium of the sensory lines. 



In the 100-hour stage (fig. 3) the ectodermal thickening has 

 changed little, but the actual contact is 100 n in length. The 



