THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



201 



is the anlage of the right ventricle, the left part, of the left ventricle. At the 

 same time the atrial portion has moved still farther cranially so that it lies 

 to the cranial side of the ventricular portion. The venous and arterial 

 ends of the heart have thus reversed their original relative positions. At 

 this point it should be noted that the atrial end of the heart is connected 

 with the large venous trunk formed by the union of the omphalomesenteric 

 veins and the ducts of Cuvier the sinus venosus. 



During the changes in the heart as a whole, certain changes also occur in 

 the endothelial and muscular walls. The walls of the atria are composed 

 of compact plates of muscle with the endothelium closely investing the inner 

 surface. The walls of the ventricular portion, on the other hand, become 

 thicker and are composed of an outer compact layer of muscle and an inner 

 layer made up of trabeculae which are closely invested by the endothelium. 



Septum spurium 



Atrial septum 

 (septum superius) 



Opening of sinus venosus 



Right atrium 

 Left atrium 

 Atrio-ventricular canal 



Right ventricle 

 Ventricular septum 

 Left ventricle 



FIG. 171. Dorsal half of heart (seen from ventral side) of a human embryo of 10 mm. His. 



Everywhere the endothelium is closely applied to the inner surface of the 

 myocardium, the space which originally existed between the endothelium 

 and mesothelium being obliterated. 



The embryonic heart in Mammals in the earlier stages resembles that of the adult in 

 the lower Vertebrates (Fishes). The atrial portion receives the blood from the body veins 

 and conveys it to the ventricular portion which in turn sends it out through the arteries 

 to the body. The circulation is a single one. This condition changes during the foetal 

 life of Mammals with the development of the lungs. The same transition occurs in the 

 ascending scale of development in the vertebrate series in those forms in which gill breath- 

 ing is replaced by lung breathing. The change consists of a division of the heart and 

 circulation, so that the single circulation becomes a double circulation. In other words, 

 the heart, is so divided that the lung (pulmonary) circulation is separated from the 

 general circulation of the body. This division first appears in the Dipnoi (Lung Fishes) 

 and Amphibians in which gill breathing stops and lung breathing begins, although here 



