THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 561 



free edge is directed upwards and grows toward the bulbus arteriosus 

 and the atrioventricular opening. The latter originally lies more in 

 the left half of the ventricle (fig. 310 A F.av.c), but it gradually 

 moves over more to the right, and finally assumes such a position 

 that the ventricular partition by its growth upwards meets it exactly 



Pu s Ao Oi 



Fig. 310. Two diagrams (after BORN) to elucidate the changes in the mutual relations of the 

 ostium atrioventriculare and the ostium interventriculare, as well as the division of the 

 ventricle and large arteries. The ventricles are imagined to have been divided into halves ; 

 one looks into the posterior [dorsal] halves, in which, moreover, the cardiac trabeculae, etc., 

 have been omitted for the sake of simplifying the view. 



A, Heart of an emhryo Rabbit, in which the head is 3 - 5 5-8 mm. long. The ventricle is 



divided by the ventricular partition (ks) into a left and a right half as far as the ostium 

 interventriculare (Oi). The right end of the foramen atrioventriculare commune (F.av.c) 

 extends into the right ventricle ; the endocardial cushions (o.ek, u.ek) are developed. 



B, Heart of an embryo Rabbit, head 7'5 mm. long. The endocardial cushions (o.ek, u.ek) of the 



foramen atrioventriculare commune are fused, and thereby the for atrioventr. com. is now 

 separated into a for. atrioventr. dextrum (F.av.d) and sinistrum (F.av.s). The ventricular 

 partition (ks) has likewise fused with the endocardial cushions, and has grown forward as far 

 as the partition (s) of the truncus arteriosus. By the closure of the remnant of the ostium 

 interventriculare (Oi) the septum membranaceum is formed. 



rk, Right, Ik, left ventricle ; ks, ventricular partition ; Pu, arteria pulmonalis ; Ao, aorta ; 

 s, partition of the truncus arteriosus ; Oi, ostium interventriculare ; F.av.c, foramen atrio- 

 ventriculare commune ; F.av.d and F av.s, foramen atrioventriculare dextrum and sinistrum ; 

 o.ek, u.ek, upper and lower endothelial or endocardial cushions. 



in the middle and fuses with its edges directly opposite the atrial 

 partition (figs. 309, 310 B). 



The division of the ventricle in Man is completed as early as the 

 seventh week. From the atrium, the two compartments of which 

 are united by the foramen ovale, the blood is now conducted through 

 a right and a left ostium atrioventriculare into completely separated 

 right and left ventricles. 



The two atrioventricular openings are narrow at the time of 

 their origin ; they are in part surrounded by the previously mentioned 



36 



