DEVELOPMENT OP PELVIC GIRDLE IN THE CHICK. 21 



birds, and Mammals, and the pubis itself is in both cases homo- 

 logous, or (2) the pubis of reptiles is the pectineal process, and 

 the processus lateralis is the pubis of the higher forms. 

 The first is the view apparently assumed by Huxley.^ Sup- 

 posing it to be true, the processus lateralis, in becoming the 

 pectineal process, has retained the forward and outward direc- 

 tion which it has in the Chelonia. In Dinosaurs the down- 

 ward direction is also seen. The pubis itself has become 

 rotated backwards. The mere fact of its pointing forwards in 

 reptiles and backwards in Dinosaurs, Birds, and Mammals, is 

 no reason whatever against the theory of its being homologous 

 in the two cases, for it is generally believed that the whole 

 girdle has rotated in Mammals through an angle of about 90° 

 from the position it occupies in reptiles. This would com- 

 pletely account for the altered position of the pubis. The fact 

 that the angle formed at the symphysis of the pubes has gene- 

 rally become more acute in Mammals is a natural consequence 

 of the transition from the crawling flat-bodied reptiles to the 

 higher walking forms, in which the body is more laterally 

 compressed. 



In birds the case is somewhat different. The fact of the two 

 primary sacral vertebrae being situated, as Gegenbaur^ has 

 shown, at a very short distance behind the acetabulum may in- 

 dicate that the girdle has been rotated backwards to some 

 extent from the reptilian position. The pubis may thus have 

 come to point vertically downwards or very slightly backwards, 

 as in the embryo bird. The adductor muscles passing from the 

 pubis and ischium to the femur in reptiles are to a great extent 

 Replaced in birds by large muscles, which act as flexors of the 

 thigh and adductors of the leg. It is evidently advantageous 

 for these muscles to arise high up, and for their points of origin 

 td be as rigid as possible. These advantages are attained by 

 the disposition of the bones in the adult bird's pelvic girdle, the 



' Huxley, " On the Pelvis in Mammalia," ' Proceedings of Koyal Society,' 

 Vol; xxviii, 1879. 



' Gegenbaur, " Beitrage zur Kenntniss des Beckens der Vogelj" ' Jenaische 

 Zeitschrift,' Band vi, 1871. 



