THE ECTODERM AND ITS DERIVATIVES 



175 



UPPER EYELID 



CIUARY BODY 



EPIDERMIS 



SUSPENSORY LIGAMENT 



OUS HUMOR 

 ORBITAL CAVITY 



( CAPSULE 

 LENS { EPITHELIUM 

 ( FIBERS 



NICTITATING MEMBRANE 

 LOWER EYELID 



PTIC NERVE 



SCHEMATIC DIAGRAM THROUGH FROG'S EYE 



(REDRAWN FROM MANGOLD '31) 



gives rise to the connective tissue of the choroid coat that surrounds 

 the pigmented layer of the eye. Outside of the choroid coat is the very 

 tough sclerotic coat, also mesenchymal. The nervous (sensory) parts 

 of the eye are therefore ectodermal in origin, but the blood vessels, 

 connective tissues, cartilage, and parts of the cornea are all mes- 

 enchymal (mesoderm). 



The Ear. 



The frog has no outer ear. The inner and the middle ear are de- 

 veloped much in the manner of all vertebrate ears, but to a less efficient 

 and complicated degree. 



The Inner Ear. The superficially placed auditory placode de- 

 velops from nervous ectoderm on the side of the head at the level 

 of the rhombencephalon, prior to the time of hatching (at about the 

 2.5 mm. stage). This occurs under the inductive influences of the 

 medulla and archenteric roof. It moves inwardly toward the brain and 



