THE SKELETON 



439 



In the chick at a corresponding age the ilium is much: more 

 extensive, and the ischium is united with it by cartilage; the 

 pubis, however, has only a membranous connection with the 

 ilium (contra Johnson). In the course of development the distal 

 ends of the ischium and pubis rotate backwards until the two 

 elements come to lie substantially parallel to the ilium (Figs. 

 242 and 249). The displacement of the ischium and pubis may 



Cr.fS. 





S.s. 



a&.M 



FIG. 248. Sagittal section of the right half of the body 

 of Larus ridibundus, to show the composition of the pel- 

 vic girdle; x 35. Length of the leg-bud of the embryo, 

 0.4 mm. (After Mehnert.) 



F., Femur, cr. N., Crural nerve. II., Ilium. I. s., Is- 

 chium. Is. N., Ischial nerve, ob. N., Obturator nerve. 

 P., Pubis. 



be associated with the upright gait of birds; it is fully established 

 on the eighth day in the chick. The mode of ossification, which 

 is perichondral, is shown in Fig. 249. 



Later, the ilium obtains a very extensive pre- and post- 

 acetabular union with the vertebrae. I have found no evidence 

 in a complete series of preparations (potash) of attachment by 

 ribs arising as independent ossifications. The ischium also fuses 



