438 THE DEVELOPMENT OF THE CHICK 



III, IV. Metacarpal II is only about one third the length of III. 

 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 

 I and 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, III, 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 

 cartilage centers in the common mass (usually), or as separate 

 centers of ossification in a common cartilaginous mass (ilium 

 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. 



