THE SKELETON 



441 



fibula. The fibular cartilage extends the entire length of the crus, 

 but ossification is confined largely to its proximal end; on the 

 fourteenth day its lower half is represented by a thread-like fila- 

 ment of bone. 



No separate tarsal elements are found in the adult; but in the 

 embryo there are at least three cartilages, 

 viz., a fibulare, tibiale and a large distal 

 element opposite the three main metatar- 

 sals. In the course of development, the 

 two proximal elements fuse with one 

 another, and with the distal end of the 

 tibia. The distal element fuses with 

 the three main metatarsals, first with the 

 second, then with the fourth, and lastly 

 with the third (Johnson). 



Five digits are formed in the mem- 

 branous stage of the skeleton. In the 

 case of the fifth digit, only a small nodule 

 of cartilage (fifth metatarsal) develops and 

 soon disappears. The second, third, and 

 fourth are the chief digits; the first is 

 relatively small. Metatarsals 2, 3, and 4 

 are long and ossify separately in a peri- 

 chondral fashion. They become applied 

 near their middle and fuse with one 

 another and with the distal tarsal element 

 to form the tarso-metatarsus of the adult 

 (Fig. 250). The first metatarsal is short, 

 lying on the preaxial side of the distal end 

 of the others (Fig. 249); it ossifies after 

 the first phalanx. The number of pha- 

 langes is 2, 3, 4, and 5 in the first, second, third, and fourth digits 

 respectively (Fig. 249). 



The patella is clearly seen in potash preparations of thirteen-day 

 chicks. At the same time there is a distinct, though minute, separate 

 center of ossification in the tarsal region (Fig. 249). 



FIG. 250. Photograph 

 of the skeleton of the 

 foot of a chick embryo 

 of 15 days' incubation. 

 (Preparation and pho- 

 tograph by Roy L. 

 Moodie ) 



1, 2, 3, 4, First, second, 

 third, and fourth digits. 

 M 2, M 3, M 4, Second, 

 third, and fourth meta- 

 tarsals. 



