60 GENERAL ZOOLOGY 



sphenethmoid of the cranium are cartilage bones, by which is 

 meant that they are first laid down in cartilage, and later con- 

 verted into bone. The remaining cranial bones, the fronto- 

 parietal, parasphenoid, nasal, and palatine bones are membrane 

 bones, i.e., they develop directly out of connective tissue and are 

 not preformed in cartilage. If they are carefully removed, the 

 cartilaginous cranium is found beneath them enclosing the brain 

 completely except for several openings in the roof. 



It has already been pointed out that the core or axis of the 

 lower jaw is formed by Meckel's cartilage. The upper jaw like- 

 wise is built on a cartilaginous basis continuous with the chon- 

 drocranium. Excepting the quadratojugal and mentomeckelian 

 bones, the components of the suspensory apparatus and the 

 jaws are membrane bones. 



Some of the general and more obvious differences between 

 the human skull and that of the frog are shown in Fig. 33. In the 

 human skull there has been an enormous increase in size of the 

 cranium as compared with the frog. The frontoparietal region 

 of the frog is flat; in the human skull it is highly elevated and 

 convex. The human cranium is also relatively wider and more 

 elongated posteriorly above the occipital region. The human 

 jaws are relatively shorter than those of the frog. The human 

 skull is completely ossified. The human face results from the 

 increased size of the cranium accompanied by a relative and 

 actual reduction in visceral skeleton. 



Vertebral Column. — The vertebral column or backbone of the 

 frog consists of a series of nine vertebrae, all of which are based on 

 a common structural plan, followed by a tenth, the rodlike 

 urostyle. The urostyle represents the caudal vertebrae of the 

 frog tadpole which become fused into a single bone during the 

 process of metamorphosis. Each of the first nine vertebrae, 

 except the first and ninth, consists of (1) a centrum, a solid ventral 

 portion, concave in front and convex behind, and (2) a neural 

 arch, forming the dorsal half of the vertebra (Fig. 35). The 

 space between the arch and the centrum is occupied by the 

 spinal cord. On the dorsal side of the arch in the mid-line is a 

 projection, the neural spine. From either side of the neural arch 

 a transverse process extends laterally. In a fresh specimen each 

 transverse process bears at its outer end a short cartilaginous 

 segment, which represents a rudimentary rib. Ribs are there- 



