THE DEVELOPMENT OF THE VASCULAR SYSTEM. 227 



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 com- 

 pact 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 



Right atrium If] IJmS^ / Left atrium 



Right ventricle [ Left ventricle 



Interventricular furrow 

 FIG. 199. Ventral view of heart of human embryo of 5 mm. His. 



trabeculae which are closely invested by the endothelium. 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 coi dition changes during the fcetal 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 breathing 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 the division is not complete. In Reptiles 

 the division is complete except for a small direct communication between the ventricles. 



