PLATE 4 



EXPLANATION' OF FIGURE.- 



7 Shows the detail of the first gill thickening in a 76-hour embryo. The sec- 

 tion lies anterior to that of figure 27 just behind the area of contact of the first 

 endodermic gill pocket. X 300. 



8 From a 100-hour embryo, shows the primordium of the supra-orbital lines, 

 and its relation to the posterior extension of the epibranchial placode. X 300. 



9 to 12 Drawn from an embryo of 137 hours, in which the development was 

 below normal, so that they illustrate conditions prior to the 120^-hour stage. X 

 300. 



9 Drawn through the primordium of the lines, and shows the characteristic 

 cell arrangement. 



10 Drawn from the next section anterior to figure 9, and exhibits the first 

 appearance of the bifurcation of the anterior end of the primordium. 



11 Taken two sections anterior to figure 10, and is the first section to show 

 a complete bifurcation. 



12 Drawn from the next section anterior to figure 11. This shows the two 

 lines (supra-orbital and sub-orbital) are diverging at this point, as will be seen 

 by comparing figures 11 and 12. 



624 



