438 THE DEVELOPMENT OF THE CHICK 
Ill, IV. Metacarpal I is only about one third the length of IIT. 
Metacarpal IV is much more slender than III, and is bowed out 
in the middle, meeting III at both ends. The elements are at 
first distinct, but II and III fuse at their proximal ends in the 
process of ossification. Cartilaginous rudiments of metacarpals 
Land V have also been found by Parker, Rosenberg, and Leighton. 
As to the phalanges, Parker finds two cartilages in II, three 
in III, and two in IV on the seventh day; but already on the 
eighth day the distal phalanges of III and IV have fused with 
the next proximal one. 
As regards the homology of the digits of the wing, the author has 
adopted the views of Owen, Mehnert, Norsa, and Leighton, that they 
represent numbers II, HI, and IV, which seem to be better supported 
by the embryological evidence than the view of Meckel, Gegenbauer, 
Parker, and others, that they represent I, II, and III. 
The Skeleton of the Hind-limb. The skeleton of the hind- 
limb and pelvic girdle develops from a continuous mass of mesen- 
chyme situated at the base of the leg-bud. The original center 
of the mass represents the acetabular region; it grows out in four 
processes: (1) a lateral projection in the axis of the leg-bud, the 
primordium of the leg-skeleton proper, (2) a dorsal process, the 
primordium of the ilium; and two diverging ventral processes, 
one in front of the acetabulum (3) the pubis, and one behind 
(4) the ischium. In the membranous condition the elements are 
continuous. The definitive elements develop either as separate 
‘artilage centers in the common mass (usually), or as separate 
centers of ossification in a common cartilaginous mass (lium 
and ischium). 
The Pelvic Girdle. The primitive relations of the elements of 
the pelvic girdle in Larus ridibundus is shown in Fig. 248, which 
represents a section in the sagittal plane of the body, and thus 
does not necessarily show the full extent of any of the cartilagi- 
nous elements, but only their general relations. The head of the 
femur is seen in the acetabulum, the broad plate of the ilium 
above and the pubis and ischium as cartilaginous rods of almost 
equal width below, the pubis in front and the ischium behind 
the acetabulum. In this stage the pelvic girdle, in this and 
many other species of birds, consists of three separate elements 
on each side in essentially reptilian relations. 
