52 THE STUDY OF CHICK EMBRYOS 



is differentiated into a thick walled pouch on each side lateral to the endothelial 

 layer of the veins. 



Transverse Section through the Heart (Fig. 36). As we pass cephalad in 

 the series of sections the vitelline veins open into the heart just in front of the 

 fovea cardiaca. The entoderm in the head-fold now forms the crescentic pharynx 

 or fore-gut separated by the heart and splanchnic mesothelium 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, similar to those which 

 form the walls of the vitelline veins in the preceding sections. The median walls 

 of these tubes disappear at a slightly later stage to form a single tube. Thick- 



Ectoderm 



Somatic mesoc/er 

 Notoehord 



Myocardium 

 Entoderm 



'eural tube 



Descending aorta 



Splanchnic mes. 



FIG. 36. Transverse section through the heart of a twenty-five-hour chick embryo. X 90. 



ened 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 ventral line,, the layers of splanchnic mesoderm of each side have fused, 

 separated from the splanchnic mesothelium of the germinal disc and thus the two 

 pleuro-pericardial cavities are in communication. The mesothelial wall of the 

 heart forms the myocardium and epicardium of the adult. Dorsally, the splanch- 

 nic mesoderm is continuous with the somatic mesoderm and forms the dorsal 

 mesocardium. 



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

 the heart arises as a pair of endothelial 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-walled tube of mesoderm. The origin of the endothelial cells of the 

 heart is not surely known. They may be split from the entoderm, or arise from the mesoderm. 

 According to another view, the endothelium arises in the vascular area and grows into the body 



