306 



THE RISE OF ANIMAL LIFE 



occipital # >t ■ ■f'^pt^ , 

 ,. ..1 V oorietol 



nasal 



premaxilla 



Fig. 13-30. Comparative study of the sl<ulls of frog and man. 



directed ilium joins posteriorly with the 

 ischium and ventrally with the pubis (the 

 latter remains as cartilage ) to form each half 

 of this girdle. 



The front and hind legs of the frog are 

 homologous, that is, they are very similar, 

 possessing approximately the same bones 

 although of somewhat different proportions. 

 A single bone, the humerus, which fits into 

 a cavity ( glenoid fossa ) of the pectoral gir- 

 dle, forms the top of the front leg; this is 

 followed by a pair of bones, the radius 

 and ulna, which are fused together in the 

 frog but separate in most other vertebrates. 

 The wrist is composed of several bones 

 called carpals; these are followed by the 

 metacarpals and phalanges of the digits. 

 The posterior appendage likewise has a sin- 

 gle bone, in this case called the femur, 

 wliich fits into a socket (acetabulum) in 

 tlie pelvic girdle; this bone is followed by a 

 pair of bones, the tibia and fibula, which 

 again are fused in the frog. The tarsals are 

 next, and two of these are enlarged to add 

 a joint in the hind legs, thus facilitating 

 jumping. Following the tarsals are the meta- 

 tarsals and finally the phalanges. The bones 

 of these appendages have remarkably simi- 

 lar counterparts in tlie human skeleton. 



The anterior end of the spinal column 

 articulates with the base of the skull. This 



skull is no longer a primitive and general- 

 ized t}'pe. Fusion of the many bones found 

 in fishes has taken place to such an extent 

 that some of them have been entirely lost in 

 the long evolution to the amphibian type 

 of skull. Above the level of sharks there are 

 two types of bones present in all skulls: re- 

 placement bone, which is that bone replac- 

 ing cartilage as the individual develops, and 

 dermal bone, produced from the dermis. 

 The frog skull is made up almost entirely of 

 dermal bones, and the only replacement 

 bones present are those immediately sur- 

 rounding the brain. 



The skull has been used to trace the ori- 

 gin of the amphibians as well as other types 

 of animals. It has generally been thought 

 that amphibians have given rise directly to 

 the mammals; this idea is based on the fact 

 that both mammalian and amphibian skulls 

 possess two condyles ( bony projections ) on 

 either side of the large opening, the fora- 

 men magnum, through which the cord 

 passes at the base of skull. Recently, how- 

 ever, it has been discovered that primitive 

 amphibians, like fish, have but one condyle. 

 This simply means that both mammals and 

 amphibians have followed similar paths in 

 their evolution. 



In addition to that portion of tlie skull 

 which protects the brain, there are the parts 



