DEVELOPMENT OF THE HEART. 



Fig. 169. ANTERIOR AND POSTERIOR ASPECT OF THE HEART OF A SOMEWHAT OLDER RABBIT EMBRYO. 



(Born. ) 



p a pulmonary artery ; s.r, s I, s.tr, right and left liorns and transverse part of sinus respectively ; 

 rs'.c'l.s.c, right ard left superior caviB ; i>e, aperture of pulmonary vein ; h.v, hepatic veins; <*.*, 

 ductus venosus ; me, mesocardium posterius. 



The other letters as in fig. 168. 



Fig. 170. SECTION THROUGH THE HEART OF A RABBIT EMBRYO AT THE STAGE SHOWN IN FIG. 169. 



(Born.) 



r.s, l.s, right and left horns of sinus receiving from above the respective superior venae cavae ; r.au, 

 l.au, right and left auricles ; r.v, l.v, right and left parts of the ventricle ; r.v v, l.v.v, right and left 

 valves guarding the orifica from the right horn of the sinus into the right auricle ; au.v.c, one of the 

 two endocardial cushions which are beginning to sub-divide the common auriculo-ventricular aperture. 

 The dotted line encloses the extent of the endccardial thickening ; s', first septum superior growing 

 down between th? auricles and prolonged below by a thickening of endocardium. Close to this septum 

 in the left auricle is seen the opening of the pulmonary vein ; s.inf, inferior septum of the ventricles. 



Fig. 171. YlEW FROM BEHIND OF THE HEART OF A HUMAN EMBRYO OF ABOLT 4 WEEKS, MAGNIFIED. (His.) 



^ The two superior cavse, right and left, and the inferior cava are seen opening separately into the 

 sinus which is a transversely elongated sac communicating only by a narrow orifice with the right 

 auricle. 



