DEVELOPMENT OF THE CIRCULATORY SYSTEM. 



233 



the yolk sac by a vitelline vein ; each pumps its blood into an 

 aortic stem which terminates as the artery of the yolk sac. 



pericardiac part of coelom 



,mesobtast /medullary plate 



.epiblast 



perwar. 



cardiac tube 



cardiac tube 



Fig. 191.— Transverse section of the Blastoderm showing the Right and Left Cardiac 

 Tubes situated in the Splanchnopieure. 



These two tubes become fused together to form a simple tubular 

 heart by the 3rd week. The cardiac tubes unite as the splanchno- 

 pleures come together (see Figs. 191 and 192). The lining 

 membrane of the cardiac tubes is probably furnished by an inva- 

 gination of the hypoblast (Fig. 191). The heart is at first 



neural canal 



mesoblast 



splanchno.. I /^- ? r^rS£S. * ... 

 ■pleure sUhS^*\ fore-gut 



left dorsal aorta 

 somatopleure 



L 



dorsal meso-card. 



ventral meso-card. 



Fig. 102. — Transverse section at a more advanced stage showiDg the union of the 

 Splanchnopleures to form the Mesocardia and the fusion of the Right and Left 

 Cardiac Tubes. 



suspended by a mesentery, which stretches from the fore-gut 

 (Fig. 192) to the ventral median line of the body wall and 

 separates the anterior end of the coelom into right and left 

 halves. The mesentery is formed out of the fused splanchno- 



