STRUCTURE OF THE VERTEBRATES 



165 



Further evolution is merely modification of this primitive plan. 

 The pubic bones have developed along two main lines: (1) the 

 primitive reptilian type with the pubic union, or syiyiphysis, in 

 an anterior position; and (2) the bird-like modification with 

 the pubes growing posteriorly, parallelling the ischia. The mam- 

 mal-like reptiles and mammals retained the primitive condition. 



Homology of Parts. The girdles evidently arose independ- 

 ently of each other, and homology of parts is consequently dif- 

 ficult. The appendages, on the other hand, arise embryologically 

 in the same manner and all the evidence points to a complete 

 homology of parts. 



The primitive tetrapod limbs are thought of as being held in 

 a straight line at right angles to the vertebral column. Each 

 limb had a joint between the upper and lower limb, the elbow 

 of the pectoral limb, and the knee of the pelvic. The front leg 

 was then rotated forward and under the body, and the elbow 

 was forced posteriorly as the arm was flexed. Exactly the oppo- 

 site occurred in the hind leg. The foot was carried posteriorly 

 under the tail, and the knee joint was forced anteriorly. The 

 limb position indicates the function. The front legs primitively 

 act as a support of the head, and the pelvic limbs push the 

 body along. The development of the pelvis and its appendages 

 is in correlation with the greater function of me limbs as a loco- 



