EYES OF INTERMEDIATE TYPES 171 



degree of development of the sense-organs in the larval state as compared 

 with the adult animal; also the degenerate condition or absence of 

 the same organs in the adult protochordate animal, as compared with those 

 adult vertebrates in which the lateral eyes are certainly functional and 

 also those, such as the cyclostomes and certain reptiles, in which the 

 median eyes show a high degree of structural differentiation, which 

 warrants the assumption that these median eyes also were functional in the 

 ancestral stock from which the cyclostomes and reptiles have descended. 



