236 THE BIOLOGY OF THE FROG chap. 



developed from membranes. The latter are more superficial 

 in position and may be stripped off from the rest of the skull, 

 leaving a sort of cartilaginous box with ossifications here and 

 there which represent the so-called cartilage bones. In the 

 cranium only the exoccipitals, the prootics, and the ethmoid 

 are cartilage bones, the fronto-parietal, parabasal, nasals, 

 and palatines are developed from membranes, and may 

 readily be separated from the underlying cartilage. The 

 cartilaginous cranium forms an almost complete case for the 

 brain ; the roof, however, is incomplete, there being a large 

 oblong opening, or fontanelle, the fenestra frontalis, near 

 the middle, and a pair of smaller openings, fenestras paiie- 

 ta/es, farther back. 



The suspensortum and jaws also have a cartilaginous basis 

 which is continuous with that of the cranium proper. The 

 bones of these parts, with the exception of the quadrato- 

 jugal and the mento-meckelian bones, are developed from 

 membrane. A cartilaginous bar runs between the pterygoid 

 and tympanic to the angle of the upper jaw, whence it is 

 continued forward beneath the maxillary as far as the pala- 

 tine bone, where a transverse process joins the posterior end 

 of the nasal capsule. The exposed cartilage on the side of 

 the cranium is perforated by a large foramen for the optic 

 nerve and by smaller openings for the third and fourth 

 nerves. The angulare and dentary of the lower jaw are 

 membrane bones applied to the cartilaginous core, or 

 Meckel's cartilage. The thyroid processes of the hyoid are 

 developed from cartilage. 



The frog's skull represents a type between the skulls of 

 the lower fishes and those of the higher vertebrates. In 

 such forms as sharks and skates the cranium and its various 

 appendages are entirely composed of cartilage. Higher up 

 in the scale we meet with fishes, such as some of the ganoids. 



