THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



225 



While the double origin of the heart is characteristic of all amniotic Vertebrates (Reptiles, 

 Birds, Mammals), in all the lower forms the organ arises as a single anlage. In the region 

 of the fore-gut the two halves of the ccelom are separated by a ventral mesentery which 

 extends from the gut to the ventral body wall, and which is composed of two layers of mesothe- 

 lium with a small amount of mesenchyme between them. In the mesenchyme a cavity 



Oral fossa 



Ventral aortic 

 trunk 



Ventricle< 



Ant. cardinal vein 

 Duct of Cuvier 

 Umbilical vein 



Ventricle 



Atrium 



Diaphragm 



Duct of Cuvier 



Liver 



Duct of liver 



FIG. 196. Ventral view of reconstruction of human embryo of 2.15 mm. His. 



The ventral body wall has been removed. The vessels (in black) at the sides of the duct 



of the liver are the omphalomesenteric veins. 



appears and is lined by a single layer of flat (endothelial) cells. This cavity extends longi- 

 tudinally for some distance in the cervical region and with its endothelial and mesothelial 

 walls constitutes the simple cylindrical heart. On the dorsal side it is connected with the 

 gut by a portion of the mesentery which is called the dorsal mesocardium; on the ventral 

 side it is connected with the ventral body wall by the ventral mesocardium (Fig. 197). Thus 



Entoderm 

 Mesoderm (visceral) 



Dorsal mesocardium 



Heart 



Pericard. cavity 

 (coelom) 



Endothelium 

 Mesoderm (parietal) 

 Ventral mesocardium 

 Ectoderm 



FIG. 197. Ventral part of transverse section through the heart region of Salamandra 

 maculosa embryo with 4 branchial arches. Rabl. 



the heart is primarily a single structure. The difference between the two types of develop- 

 ment is not a fundamental one but simply depends upon the difference in the germ layers. 

 In the lower forms the germ layers are closed in ventrally from the beginning, and the heart 

 appears in a medial position. In the higher forms the germ layers for a time remain spread 

 out upon the surface of the yolk or yolk sac, and the heart begins to develop before they 

 close in on the ventral side of the embryo. Consequently the heart arises in two parts which 

 are carried ventrally by the germ layers and unite secondarily. 



