. 



THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



229 



is guarded, as it were, by a lateral and a medial fold the significance of which will 

 be described later. The ventricular portion also shows a ridge projecting from 

 the caudal side, which corresponds to the interventricular groove and represents 

 the beginning of the 'ventricular septum. 



The Septa. The further changes are largely concerned with the separation 

 of the heart into right and left sides, and with the development of the valves. 

 The atria become separated by the further growth on the cephalic side, of the 

 ridge which has already been mentioned and which is known as the septum 

 superius' (Figs. 200 and 201). This septum grows across the cavity of the atria 

 until it almost reaches the a trio- ventricular canal, forming the septum atriorum. 

 A portion of the septum then breaks away, leaving the two atria still in com- 



Sinus xenosus 

 Left valvula venosa 

 Right valvula venosu 

 Right 



Atrial septum 



Pulmonary vein 



Left atrium 



Right atrio- 



ventricular canal " 



Right ventricle -' 



Left atrio- 

 ventricular canal 



Left ventricle 



Interventricular furrow 



Ventricular septum 



FIG. 202.- Dorsal half of heart (ventral view) of rabbit embryo of 5.8 mm. Born. 



munication. This secondary opening is the foramen ovale which persists 

 throughout foetal life but closes soon after birth. The atrio-ventricular canal 

 also becomes divided into two passages by a ridge from the dorsal wall and 

 one from the ventral wall uniting with each other and finally with the septum 

 atriorum (Fig. 201). Thus the two atria would be completely separated if it 

 were not for the foramen ovale. 



During the separation of the atria, a division of the ventricular portion of the 

 heart also occurs. On the caudal side of the ventricular portion a septum ap- 

 pears and gradually grows across the cavity forming the septum ventriculorum 

 (Figs. 200 and 201). Thi septum is situated "nearer the right side and is in- 

 dicated on the outer surface by a groove which becomes the sulcus longitudlnalis 



