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



where it is separated by unmodified ectoderm from the second 

 primordium, preclude such an idea. Even in the case of the 

 last two, there is a sufficient amount of unmodified ectoderm be- 

 tween them in the first series in which they appear to make it 

 extremely improbable that they were ever continuous, although 

 it is a possibility. 



SUMMARY AND DISCUSSION 



The preauditory region 



1. The lateral line primordia arise in Lepidosteus quite dis- 

 tinct from the auditory vesicle, but so situated with reference to 

 other thickenings associated with the gills, both the hyoid and 

 the true gills, that all these structures must be followed in a close 

 series to insure their differentiation. 



2. In the preauditory region there are the following structures 

 to be differentiated : (a) The preauditory placode or anterior ex- 

 tension of the auditory vesicle; (b) The thickening of the ecto- 

 derm at the point where the endodermic gill pocket of the hyoid 

 gill comes into contact with the ectodem; (c) The posterior ex- 

 tension of b which grows back of the area of contact of the en- 

 dodermic pocket with the ectoderm. In this posterior extension 

 develops the epibranchial placode; (d) The epibranchial placode 

 which appears after a and b and before c and on account of its 

 size and character is not a particular source of difficulty; (e) 

 The lateral line primordia; these appear after a and b and con- 

 sist of a common primordium for the supra- and infra-orbital 

 lines and one for the mandibular line. 



3. The preauditory placode appears approximately at the same 

 time as b, the ectoderm gill thickening. It is the anterior ex- 

 tension of the auditory vesicle, resembling it in histological struc- 

 ture but is smaller in dorso-ventral diameter. Before a cavity 

 appears in the vesicle it is difficult to locate the anterior end of 

 the vesicle on account of a gradual transition of the vesicle into 

 the placode. In the early stages the anterior end of the preau- 

 ditory placode extends forward to the hyoid region and abuts 



