6o8 PECTORAL AND PELVIC GIRDLES. 



considered to have a share in forming it. The distal ends of the pubis 

 and ischium are close together when first formed, but subsequently separate. 

 Each of them unites at a late stage with the corresponding process of the 

 opposite side in a ventral symphysis. A centre of ossification appears in 

 each of the three processes of the primitive cartilage. 



Aves. In Birds the parts of the pelvic girdle no longer develop as a 

 continuous cartilage (Bunge). Either the pubis may be distinct, or, as in the 

 Uuck, all the elements. The ilium early exhibits a short anterior process, 

 but the pubis and ischium are at first placed with their long axes at right 

 angles to that of the ilium, but gradually become rotated so as to lie paral- 

 lel with it, their distal ends pointing backwards, and not uniting ventrally 

 excepting in one or two Struthious forms. 



Mammalia. In Mammalia the pelvic girdle is formed in cartilage 

 as in the lower forms, but in Man at any rate the pubic part of the carti- 

 lage is formed independently of the remainder (Rosenberg). There are 

 the usual three centres of ossification, which unite eventually into a single 

 bone the innominate bone. The pubis and ischium of each side unite with 

 each other ventrally, so as completely to enclose the obturator foramen. 



Huxley holds that the so-called marsupial bones of Monotremes and 

 Marsupials, which as shewn by Gegenbaur (No. 474) are performed in carti- 

 lage, are homologous with the praepubis of the Urodela ; but considering 

 the great gap between the Urodela and Mammalia this homology can only 

 be regarded as tentative. He further holds that the anterior prolongations 

 of the cartilaginous ventral ends of the pubis of Crocodilia are also struc- 

 tures of the same nature. 



BIBLIOGRAPHY. 



(473) A. Bunge. Untersuch. z, Entwick. d. Beckengiirtels d. Amphibien, 

 Reptilien u. Vogel, Inaug. Diss. Dorpat, 1880. 



(474) C. Gegenbaur. " Ueber d. Ausschluss des Schambeins von d. Pfanne 

 d. Hiiftgelenkes." Morph. Jahrbuch, Vol. II. 1876. 



(475) Th. H. Huxley. "The characters of the Pelvis in Mammalia, etc." 

 Proc. of Roy. Soc., Vol. xxvm. 1879. 



(476) A. Sabatier. Comparaison des ceintures et des membres anterieurs et 

 posterieurs dans la Serie d. Vertebrcs. Montpellier, 1880. 



Comparison of Pectoral and Pelvic girdles. 



Throughout the Vertebrata a more or less complete serial 

 homology may be observed between the pectoral and pelvic 

 girdles. 



In the cartilaginous Fishes each girdle consists of a continuous 

 band, a dorsal and ventral part being indicated by the articulation 

 of the fin ; the former being relatively undeveloped in the pelvic 



