THE LATER DEVELOPMENT OF THE CHICK 339 



ventricle -from the bulbus arteriosus, the most anterior chamber 

 of the heart, formed from the anterior loop of the original heart 

 tube and now passing obliquely upward to the antero-dorsal 

 wall of the pericardial cavity, where it connects with the truncus 

 arteriosus in the floor of the pharynx. The wall of the bulbus is 

 also much thickened at this time. 



Long before the end of the third day the heart is beating regu- 

 larly and rapidly. Its first irregular twitching begins toward 

 the middle of the second day and before the end of this day its 

 contraction becomes quite regular. 



The later development of the heart may be sketched only 

 roughly. There are further changes in the position of the heart, 

 as the net result of which the ventricle assumes a posterior, and 

 the auricles an antero-dorsal, location; the auricles also grow 

 ventrally around the bulbus, which finally occupies a median 

 ventral position. 



The bulbus arteriosus finally loses its identity as a separate 

 chamber. Three semilunar valves develop, about in its middle, 

 and its anterior section then becomes transformed into the proxi- 

 mal parts of the truncus arteriosus (finally the root of the dorsal 

 aorta) and the pulmonary artery, while its posterior section is 

 absorbed into the ventricles. Similarly the sinus venosus be- 

 comes incorporated into the right auricle, and the sinu-auric- 

 ular valves, which had developed from the wall of the sinu- 

 auricular aperture, entirely disappear. Thus of the original 

 four chambers of the heart only two remain separate. The 

 cavities of these two chambers, the auricle and ventricle, then 

 become secondarily divided longitudinally, so that the heart 

 may again be described as four-parted, though in a very 

 different sense. 



From the postero-dorsal wall of the auricle, between the open- 

 ings of the sinus venosus and the pulmonary vein (see below) a 

 thin partition grows downward and forward, during the fourth 

 day, and soon reaches a thickened cushion of cells lying on the 

 upper and lower sides of the auriculo-ventricular aperture, thus 

 completely dividing the primary auricular cavity into right and 

 left cavities (Fig. 133, F). This interauricular septum early 



