440 



THE DEVELOPMENT OF THE CHICK 



with the ventral posterior border of the iUum, and the pubis, 



except at its anterior and posterior ends, with the free border 



of the ischium. 



The spina iliaca, a pre-acetabular, bony process of the ihum, 



requires special mention in- 

 asmuch as it has been inter- 

 preted (by Marsh) as the 

 true pubis of birds, and the 

 element ordinarily named 

 the pubis as homologous to 

 the post-pubis of some rep- 

 tiles. There is no evidence 

 for this in the development. 

 The spina iliaca develops as 

 a cartilaginous outgrowth of 

 the ilium and ossifies from 

 the latter, not from an inde- 

 pendent center (Mehnert). 



The Leg-skeleton. The 

 skeleton of the leg develops 

 from the axial mesenchyme, 

 which is at first continuous 

 with the primordium of the 

 pelvic girdle. In the process 

 of chondrification it seg- 

 ments into a larger number 

 of elements than found in 

 the adult, some of which are 

 suppressed and others fuse 

 together. The digits grow 

 out from the palate-like ex- 

 pansion of the primitive 

 limb in the same fashion as 

 in the wing. In general the 



separate elements arise in the proximo-distal order (Figs. 242 and 



249).. 



The femur requires no special description; ossification begins 



on the ninth day. 



The primordium of the fibula is from the first more slender 

 than that of the tibia, though relatively far larger than the adult 



Fig. 249. — Photograph of the skeleton 



of the leg of a chick embryo of 15 days' 



incubation. Prepared by the potash 



method. (Preparation and photograph 



by Roy L. Moodie.) 



1, Tibia. 2, Fibula. 3, Patella. 4, 

 Femur. 5, Ilium. 6, Pleurocentra of 

 sacral vertebrae. 7, Ischium. 8, Pubis. 

 9, Tarsal ossification. 10, Second, third, 

 and fourth metatarsals. 11, First meta- 

 tarsal. I, II, III, IV, First, second, third, 

 and fourth digits. 



