Plate VI. 



Fig. 21, Stage III. Showing iucTeased differentiation in the optic area, 

 and the double character of the rudiment from which the rostrum is 

 derived. 



Fig. 22. Showing some advance over Fig. 21. The cerebral ganglia, c. gh, 

 and optic ganglia, o. gl., are more distinctly dilTerentiated. the rostrum has 

 become more prominent, as is also the stomoda?al depression. 



