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



Hyp., hypophysis Po.LL. 1, 2, 3, the first, second and 



Lat.IX, lateralis ganglion of IX nerve third postauditory lateral line pri- 



Lat.X, lateralis ganglion of X nerve mordia 



Mes., mesencephalon Pr.A.P., preauditory placode 



Met., metencephalon Pr.L.L., primordium of sensory lines 



Md.L., hyo-mandibular line anterior to auditory vesicle 



Olf.C, olfactory capsule Pre, prosencephalon 



Opt.V., optic vesicle Sub.L., sub-orbital line 



P.E.P., posterior extension of epibran- Sup.L., supra-orbital line 



chial placode of VII Vis. IX, visceral ganglion of the IX 



Ph., pharynx nerve 



Po.A.P., postauditory placode Vis.X, visceral ganglion of the X nerve 

 Po.L.L., primordium of lateral lines 



posterior to auditory vesicle 



Figures 1 to 6 are flat reconstructions of Lepidosteus osseus made at a magni- 

 fication of 75 diameters. In all six figures the brain and sense organs are shown 

 in outline only. The ectodermal gill thickenings are shown in diagonal lines; the 

 actual area of contact between ectoderm and endoderm in the hyoid region by an 

 open line; the epibranchial placode by stipple; the primordia of the lateral lines 

 by cross-hatching and the preauditory placode by vertical lines. 



PLATE 1 



EXPLANATION OF FIGURES 



1 Reconstruction from Stage XVI, age 88 hours. The preauditory placode is 

 in this stage a cephalad continuation of the auditory vesicle. The ectodermal 

 thickening in the region of the hyoid contact is shown, continuous posteriorly 

 with the preauditory placode. In the postauditory region two thickenings are 

 present. The more dorsal is associated with the first gill and the more ventral 

 with the second gill. 



2 Stage XVII; age 94 hours; shading and abbreviations as in figure 1. The 

 auditory vesicle is completely detached fom the ectoderm and the preauditory 

 placode. The placode is quite small and overlaps the auditory vesicle and 

 extends three sections (SOfi) only in front of it. The epibranchial placode is 

 present and just dorsal to it lies the primordium of the supra- and infra-orbital 

 lines. In the postauditory region, the first gill thickening is completely detached 

 from the second which extends considerably farther caudad; 



