42 



THE STUDY OF CHICK EMBRYOS 



lium of the germinal disc; thus the two pleuro-pericardial cavities are put in communication.. 

 The mesothelial wall of the heart forms the myocardium and epicardium of the adult. Dor- 

 saUy, the splanchnic mesoderm, as the dorsal mesocardium, suspends the heart, while stUl 

 more dorsally it is continuous with the somatic mesoderm. 



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

 the heart arises as a pair of endothehal tubes lying in the pockets of the splanchnic mesoderm. 

 Later, the endothelial tubes fuse to form a single tube. The heart then consists of an endo- 

 thelial tube within a thick-waUed tube of mesoderm. The origin of the endothehal cells of 

 the heart— whether they arise from entoderm or mesoderm — is not surely known. T he v as- 

 c ular system is primitively a paired system, the heart arising as a double tube wi th 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 is differentiated as the^ angioblas t. 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 undergojlifferejiti ation intn two cell types, the innprmnst be com- 

 JngTlood cells, the o utermost formin g a flattened endothelial layer which enrlnses the blood 



Neural tube 



Ectoderm 



Somatic mesoderm 



Nolochor 



Myocardium 



Descending aorta 

 Pharynx 



Endocardium 



_ Splanchnic mesoderm 



Entoderm — -jfS^ 



Fig. 37. — Transverse section through the heart of a twenty-five-hour chick embrj-o. X 90 



cells. All_t he primitive blood vessels of the embryo are composed of an endoth e lial lay er 

 only. The endothehal 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 of the branches of which enlarge to form vascular trunks. 

 One pair of such trunks, th e vitelline ve ins, is differentiated adjacent to the posterior end of 

 the heart and later connects with it. Another pair, the vitelline art eries, a re developed in 

 continuation with the aortae of the embryo. The vessels of the vascular area thus appear 

 before those of the embryo have developed; they probably arise from the splanchnic meso- 

 derm, and, both arteries and veins, are composed of a simple endothelial wall. As the coelom 

 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.) 



Transverse Section through the Pharyngeal Membrane (Fig. 38). — This section 

 passes through the head fold and shows the head free from the underlying germinal disc 

 (cf . Fig. 42) . The ectoderm surrounds the head and near the mid- ventral line it is bent dor- 

 sad, is somewhat thickened, and comes in contact with the thick entoderm of the pharynx. 

 The area of contact between ectoderm and pharyngeal entoderm forms the pharyngeal plate 



