142 



EARLY EMBRYOLOGY OF THE CHICK 



root of the aorta and the pulmonary artery does not appear 

 until a later stage of development. 



During the changes in the external shape of the heart which 

 have been described, the whole heart has come to occupy a 

 more caudal position with reference to other structures in the 



H 



53 HOURS 



29 somite* 



M tomiiM 



Pig. 49. — Ventral views of the heart at various stages to show its changes 

 of shape knd its regional differentiation. All the drawings were made from 

 dissections with the aid of camera lucida outlines. The outer of the two layers 

 shown is the epi-myocardium ; the inner, the endocardium. In the stages repre- 

 sented in Figs. E-H torsion of the embryo's body is going on at the level of the 

 heart. Since torsion involves the more cephalic regions first and progresses 

 caudad the transverse axis of the body of the embryo is at different inclinations 

 to the yolk at the cephalic end and at the caudal end of the heart. In drawing 

 these figures their orientation was taken from the body at the level of the conus 

 region of the heart, the sinus region therefore appears inclined. Abbreviations: 

 a.v., constriction between atrium and ventricle; i.v., interventricular groove. 



embryo. When the heart is first formed it lies at the level of 

 the rhombencephalon. As development progresses it moves 



