EMBRYO OF SEVEN SEGMENTS 



43 



Section through the Heart (Fig. 37). Passing cephalad in the series of sections, the 

 vitelline veins open into the heart just in front of the intestinal portal. The entoderm in 

 the head fold now forms the crescentic pharynx of the fore-gut, separated by the heart 

 and splanchnic mesot helium from the entoderm of the germinal disc. The descending 

 aortae are larger, forming conspicuous spaces between the neural tube (hind-brain] and the 

 pharynx. The heart, as will be seen, is formed by the union of two endothelial tubes, con- 

 tinuous with those constituting the vitelline veins in the preceding sections. The median 

 walls of these tubes disappear at a slightly later stage to form a single tube, the endocardium. 

 Thickened layers of splanchnic mesoderm, which, in the preceding section, invested the 

 vitelline veins laterally, now form the mesothelial wall of the heart. In the median ven- 

 tral plane, the layers of splanchnic mesoderm of each side have fused and separated from 

 the splanchnic mesothelium of the germinal disc ; thus the two pleuro-pericardial cavities 

 are put in communication. The mesothelial wall of the heart forms the myocardium and 

 cpicardium of the adult. Dorsally, the splanchnic mesoderm, as the dorsal mesocardium, 

 suspends the heart, while still more dorsally it is continuous with the somatic mesoderm. 



Neural tube 



Descending aorta 



Ectoderm 



Somatic mesoderm 



Notochord 



Eft-myocardium 



___ Splanchnic mesoderm 



Entoderm- 



FIG. 37. Transverse section through the heart of a twenty-five-hour chick embryo. X 90. 



Origin of the Primitive Heart. From the two sections last described, it is seen that 

 the heart arises as a pair of endothelial tubes lying in folds of the splanchnic mesoderm. 

 Later, the endothelial tubes fuse and the mesodermal folds are also brought together. The 

 heart then consists of a single endothelial tube within a thick-walled investment of meso- 

 derm. The origin of the endothelial cells of the heart whether they arise from entoderm 

 or mesoderm is not surely known. The vascular system is primitively a paired system, 

 the heart arising as a double tube with two veins entering and two arteries leaving it 

 (cf. Figs. 268 and 269). 



Origin of the Blood Vessels and Blood. We have seen that in the area opaca a net- 

 work of blood vessels and blood islands is differentiated as the angioblast. This tissue 

 gives rise to primitive blood vessels and blood cells and probably is derived from the splanch- 

 nic mesoderm. The vessels arise first as reticular masses of cells, the so-called blood 

 islands. These cellular thickenings undergo differentiation into two cell types, the inner- 

 most becoming blood cells, the outermost forming a flattened endothelial layer which encloses 

 the blood cells. All the primitive blood vessels of the embryo are composed of an endothe- 

 lial layer only. The endothelial cells continue to divide, forming vascular sprouts and in 

 this way new vessels are in part produced. The first vessels arising in the vascular area 

 of a chick embryo unite into a close network, some bf the branches of which enlarge to 

 form vascular trunks. One pair of such trunks, the vitelline veins, is differentiated 



