FROG: EXTERNAL FEATURES AND BODY-WALL 31 



■■'u.j. 



the sphenoidal region, and the front part, immediately 

 behind the nasal capsules, is known as the ethmoidal 

 region. 



The skull consists of a foundation of cartilage taken 

 over from the tadpole, with certain bones which are 

 formed while the tadpole is changing into a frog. These 

 bones may be divided into two sets according to the way in 

 which they are formed. Those which arise by the replace- 

 ment of parts of the original cartilaginous skull by bone 

 are known as car- 

 tilage bones. Those 

 which appear in de- 

 velopment without 

 being thus pre- 

 formed in cartilage 

 are known as mem- 

 brane bones on ac- 

 count of the mem- 

 brane, consisting of 

 a sort of connective 

 tissue, which at first 

 occupies the places 

 in which they will 



appear. The car- 0. c. .ex. 



tilage bones are em- FlG _ I3 ._ The cart iia g inous skull of a 



bedded in the car- frog, seen from above after the removal 



tilage of the skull of most of the bones. 



and Cannot be re- a y^ Anterior fontanelle ; ait ., auditory capsule ; cr ., 

 mOVed, but the mem- cranium ; ex., exoccipital ; l.p.f., left posterior 



. , fontanelle; nas., nasal capsule; o.c, occipital 



Drane DOneS Can condyle ;^?-ff.,prootic;.^., pterygoid ; y., quad- 



pasilv hf> tnkpn nff rate; q.j., quadratojugal ; s$., sphenethmoid ; 



easily UC UUten Oil. «./, upper jaw bar. 



At the hind end 

 of the cranium is a large opening known as the foramen 

 magnum, through which the spinal cord is continuous with 

 the brain. On each side of this is a cartilage bone called 

 the exoccipital, which bears one of the occipital condyles 

 mentioned above, but the foramen magnum is not com- 

 pletely bordered by the exoccipitals, since these are 

 separated above and below by cartilage. The rest of the 

 cranium is mainly composed of cartilage covered by cer- 

 tain membrane bones, but the front end is formed by a 



?•,/• 



l.p.f. 



au. P r °- 



