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 
Fic. 248. 
Sagittal section of the right half of the body 
of Larus ridibundus, to show the composition of the pel- 
vie girdle; x 35. Length of the leg-bud of the embryo, 
0.4mm. (After Mehnert.) 
F., Femur. cr.N., Crural nerve. II., Ilium. I. s., Is- 
chium. Is. N., Ischial nerve. ob. N., Obturator nerve. 
Pi 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 illum obtains a very extensive pre- and_post- 
acetabular union with the vertebre. I have found no evidence 
in a complete series of preparations (potash) of attachment by 
ribs arising as independent ossifications. The ischium also fuses 
