228 



TEXT-BOOK OF EMBKYOM )( \\ . 



Fig. 200 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 cir- 

 culation but with the beginning of a separation. The atria are rather thin- 

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



Septum spurium . 



Atrial septum 

 (septum superius) 



Opening of sinus venosus 



Right atrium 

 Left atrium 

 Atrio-ventricular canal 



Right ventricle 

 Ventricular septum 

 Left ventricle 



FIG. 200. Dorsal half of heart (seen from ventral side) of a human embryo of 10 mm. His. 



atrial and ventricular portions is a canal the atrio-ventricular canal which 

 affords a free passage for the blood. From the cephalic side of the atrial por- 

 tion a ridge projects into the cavity. This ridge represents a remnant of the 

 original medial walls of the two atria and marks the beginning of the future 



Septum superius 

 Sinus venosus 



Valvulw venosse 

 Right atrium 



Right ventricle 



Ventricular septum 



Fir.. 201. Dorsal half of heart showing chambers an 

 Modified from Horn. 



srpla. 



Foramen ovale 

 Atrial septum 



- Left atrium 



Atrio-ventricular valves 



Atrio-ventricular canals 



Left ventricle 



(Semidiagrammatic.) 



air 'nil septum. The open ing of the sinus venosus is seen on the 'lorsal wall of the 

 right atrium. Primarily Loth, atria communicated directly with the sinus 

 venosus, but in the course of development the opening of the latter migrated to 

 the right and at this stage is found in the wall of the right atrium. The opening 



