PLATE 6 



EXPLANATION OF FIGURES 



17 From a 76-hoiir embi-yo; shows the detail of the preauditory placode, and 

 its resemblamce to the auditory vesicle in early stages. This figure is taken five 

 sections in front of the vesicle. X 300. 



18 Shows the auditory vesicle and the first gill thickening in a 100-hour em- 

 bryo. The overlapping of the thickening by the vesicle is shown in this section. 

 X300. 



19 Shows the detail of the preauditory placode in an 82-hour embryo. The 

 general reduction in the size of the placode and the gradual loss of radial cell ar- 

 rangement may be seen by a comparison of figures 1 and 22. X 300. 



20 From a 120|-hour embryo, shows the mandibular line. The radial ar- 

 rangement of the cells is not so apparent as in the other lines. X 300. 



21 Shows the appearance of the primordium of the supra-orbital and infra- 

 orbital lines combined in a 106-hour embryo. This section also shows that the 

 sensory line primordium lies at a more dorsal level than the posterior extension 

 of the epibranchial placode. X 300. 



22 From an 88-hour embryo, shows the appearance of the preauditory pla- 

 code. This section passes through a point two sections behind the anterior limit 

 of the placode. Compare with figures 17 to 19 which are taken at a more anterior 

 level from earlier embryos. X 300. 



628 



