200 



ESSENTIALS OF ZOOLOGY 



the lower jaw, Meckel's cartilage. The gills of the tadpole are 

 associated with a hyobranchial cartilaginous skeleton. 



About the period of metamorphosis changes take place 

 which convert the tadpole chondrocranium into a condition 

 approaching the adult. 



The chondrocranium persists to a large extent, it is only 

 partially replaced by bone. The sphenethmoid, or orbito- 

 sphenoid, is a ring-like bone which is developed around the brain 

 case in the front region of the orbit. The exoccipitals, the only 

 occipitals developed, replace the cartilage posteriorly on each 

 side of the foramen magnum. The prootics replace a large 

 part of the original auditory capsule. Another bone of great 



Nasal capsule 



Orb. proc. Aud. capsule 



* 2 3 



MeckeTs cart. Quad. Hyoid 



Stapes 



FIG. 100. Cartilaginous skull of older tadpole from the side. 



interest preceded in cartilage is the columella auris. In the 

 adult this bone lies in the cavity of the middle ear, bridging the 

 space between the tympanum and the fenestra ovalis. In the 

 tadpole it arises in association with the auditory capsule. The 

 lateral ventral aspect of the capsule remains unchondrified, 

 and the membranous space thus left is the fenestra ovalis or 

 fenestra vestibuli. It becomes narrowed and crescentic, and 

 the median part of the crescent is cut off to form a plate-like 

 cartilage occupying the membrane like a disc. This is the 

 stapes. In front of the disc an independent chondrification 

 of the fenestra gives rise to a rod-shaped cartilage which is 

 the columella. This chondrification takes place in the roof 

 of the cavity of the middle ear, which is the hyoid cleft and 

 homologous with the spiracle of the fish, and extends in the 



