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 lie 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 



FIG. 242. Photograph of the skeleton of a 13-day 

 chick embryo. Prepared by the potash method. 

 (Preparation and photograph by Roy L. Hoodie.) 

 1, Premaxilla. 2, Nasal. 3, Lachrymal. 4, Para- 

 sphenoid. 5, Frontal. 6, Squamosal. 7, Parietal. 

 8, Exoccipital. 9, Cervical rib. 10, Coracoid. 11, 

 Scapula. 12 Humerus. 13, Ilium. 14, Ischium. 15, 

 Fubis. 16 , Metatarsus. 17, Tibiofibula. 18, Pala- 

 tine. 19, Jugal. 20, Maxilla. 21, Clavicle. 



ventral extremities corresponding to the anterior end of the defini- 



ternum, which is formed by concrescence of the lateral halves 



the middle line beginning at the anterior end. The concrescence 



