164 



STRUCTURE OF THE VERTEBRATES 



form a small plate. The longer dorsal portions of the cartilages 

 strongly resemble the jemori of the land vertebrates. 



A wide gap exists, however, between any known fish and the 

 amphibia; and the anatomist is tempted to say that the true 

 pelvis developed from the simple elasmobranch type after the 

 evolution of the tetrapods. The primitive living urodeles have 

 a flat ventral cartilage with a pair of posterior centers of ossi- 

 fication, and in addition, a pair of dorsal bones which are at- 

 tached to the sacral vertebrae. The latter are the ilia. In certain 

 frogs there is an additional pair of ventral ossifications. These 



Acetabulum. 



Primitive Extinct Amphibian 

 (Stegocephalia) 



Human Embryo 



Fig. 98. Evolution of the Pelvis. The primitive pelvis (A) consisted of 

 three separate bones forming a soUd plate. The higher animals develop 

 an obturator foramen in the pubis and ischium. The mammal embryo has 

 the sutures between the three bones, although the bones ankylose during 



development. 



four ventral bones are the homologues of the anterior pubes and 

 the posterior ischia of the higher animals. But the fact that the 

 pelvic girdle of the Stegocephalia is clearly divided by sutures, 

 renders any evidence from comparative anatomy doubtful. 



The reptiles have a pelvis typical of all higher vertebrates. 

 The acetabulum is a lateral depression in which the femur 

 articulates, and is the center from which the three pelvic bones 

 radiate. The dorsal ilia attach to two sacral vertebrae, and 

 each pubis and ischium meets its mate in the mid-ventral line. 

 Thus, including the sacrum, a complete circular girdle is formed 

 which is decidedly stronger than the pectoral girdle, and con- 

 versely is less flexible. 



