426 THE DEVELOPMENT OF THE CHICK 
sions formed by the fusion of the distal ends of the first four 
true thoracal ribs; the fifth pair of thoracal ribs does not take 
part in the formation of the sternum. The sternum thus arises 
as two distinct halves, which he at first in the wall of the thorax 
at the posterior end of the pericardial cavity (eight days). The 
greatest extension of the sternal primordia is dorso-ventral, the 
Kia. 242. — Photograph of the skeleton of a 15-day 
chick embryo. Prepared by the potash method. 
(Preparation and photograph by Roy L. Moodie.) 
1, Premaxilla. 2, Nasal. 3, Lachrymal. 4, Para- 
sphenoid. 5, Frontal. 6, Squamosal. 7, Parietal. 
8, Exoccipital. 9, Cervieal rib. 10, Coracoid. 1 
Seapula. 12, Humerus. 18, Ilium. 14, Ischium. 1: 
Pubis. 16, Metatarsus. 17, Tibiofibula. 18, Pal 
tine. 19, Jugal. 20, Maxilla. 21, Clavicle. 
l 
), 
L- 
ventral extremities corresponding to the anterior end of the defini- 
tive sternum, which is formed by concrescence of the lateral halves 
in the middle line beginning at the anteriorend. The concrescence 
