THE DEVELOPMENT OF THE VASCULAR SYSTEM 



233 



the division is not complete. In Reptiles the division is complete except for a small 

 direct communication between the ventricles. 



Fig. 209 represents the dorsal half of the heart at a stage when all the 

 chambers are in open communication, and shows the conditions in a single 

 circulation but with the beginning of a separation. The atria are rather 

 thin- walled chambers, the ventricles have relatively thick walls. Between 

 the atrial and ventricular portion is a canal the atrio-ventricular canal 

 which affords a free passage for the blood. From the cephalic side of the 

 atrial portion a ridge projects into the cavity. This ridge represents a 

 remnant of the original medial walls of the two atria and marks the begin- 

 ning of the future atrial septum. The opening of the sinus venosus is seen 

 on the dorsal wall of the right atrium. Primarily both atria communicated 



Septum superius " 

 Sinus venosus 



Valvulse venossc 

 Right atrium -- 



Right ventricle / 



Ventricular septum . 



FIG. 210. Dorsal half of heart showing chambers and septa. 



Modified from Born. 



Foramen ovale 



Atrial septum 



Left atrium 



Atrio-ventricular valves 



_ Atrio-ventricular canals 

 Left ventricle 



(Semidiagrammatic.) 



directly with the sinus venosus,but in the course of development the open- 

 ing of the latter migrated to the right and at this stage is found in the wall 

 of the right atrium. The opening is guarded, as it were, by a lateral and a 

 medial fold the significance of which will be described later. The vetricular 

 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 separa- 

 tion 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. 209 and 210). This septum grows across the 

 cavity of the atria until it almost reaches the atrio-ventricular canal, form- 

 ing the septum atriorum. A portion of the septum then breaks away, leav- 

 ing the two atria still in communication. This secondary opening is the 



