EMBRYO OF SEVEN SEGMENTS 53 



of the embryo. The vascular system is primitively a paired system, the heart arising as a 

 double tube with two veins entering and two arteries leaving it. 



Origin of the Blood-vessels and Blood. — We have seen that in the area opaca a 

 network of blood-vessels and blood-islands are differentiated as the angioblast. This tissue 

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

 splanchnic 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 innermost 

 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 endothelial layer 

 only. The endothelial cells continue to divide, forming vascular sprouts and in this way new 

 vessels are produced. The first vessels arising in the vascular area of a chick embryo form a 

 close network, some of the branches of which enlarge to form vascular trunks. One pair of 

 such trunks, the vitelline veins, is differentiated opposite, and later connects with, the posterior 

 end of the heart. Another pair, the vitelline arteries, are developed in connection with the aortas 

 of the embryo. The vessels of the vascular area thus appear before those of the embryo have 

 developed, probably arise from the splanchnic mesoderm, and, both arteries and veins, are com- 

 posed of a simple endothelial wall. As the ccelom develops in the region of the vascular area 

 of the embryo soon after the differentiation of the angioblast the anlages of the blood-vessels are 

 formed only in the splanchnic layer. (For the development of the heart and blood-vessels 

 see Chapter IX.) 



Ectoderm 



Neural tube 



Afesenchyma 



Mo chord 



Pharyngeal 

 membrane—. 



Entoderm ^yy — 

 of proamnion' ■%>,. 



Fig. 37. — Transverse section through the pharyngeal membrane of a twenty-five-hour chick embryo. 



X 90. 



Transverse Section through the Pharyngeal Membrane (Fig. 37). — This 

 section passes through the head-fold and shows the head free from the underlying 

 germinal area. The ectoderm surrounds the head and near the mid-ventral line 

 is bent dorsad, somewhat thickened, and in contact with the thick entoderm of 

 the pharynx. The area of contact between ectoderm and pharyngeal entoderm 

 forms the pharyngeal plate or membrane. Later, this membrane breaks through 

 and thus the oral cavity arises. The expanded neural tube is closed in this 

 region and forms the middle brain vesicle or mid-brain. The dorsal aortae appear 

 as small vessels dorsal to the lateral folds of the pharynx. The germinal area in 

 the region beneath the head is composed of ectoderm and entoderm only. This 

 is the proamniotic area. Laterad may be seen the layers of the mesoderm. 



