FORMATION OF BLOOD VESSELS AND THE HEART 



177 



tubes develop and fuse to form the lining of the heart — the endocardium — as 

 illustrated in Figure 104. The muscular coat of the heart differentiates from 

 the splanchnic mesoderm, which fuses around the endocardium. This heart 

 begins to twitch after about 29 hours of incubation, even before the blood 

 vessels have formed a complete circulatory pathway. The heart continues to 

 beat, and at 38 hours circulation is established and the blood follows the 

 pathway indicated in Figures 101 and 105. At this early period the heart 

 does not have any nervous supply, and the rhythmic pulsations are controlled 

 entirely within the heart itself. Even if the heart is removed from the embryo 

 and cultivated in vitro it continues to contract rhythmically. Tissues from the 

 embryonic heart are very viable and can be grown in tissue culture. 



This primitive circulatory system undergoes drastic changes during de- 

 velopment. The changes, in general, follow the course of the changes which 

 occurred in the evolution of the species to which the embryo belongs. Thus, 

 in the early chick embryo we find a two-chambered heart, aortic arches, and 

 a cardinal venous system, all of which are reminiscent of the circulatory 

 system in fishes. Figures 102 and 106 show this striking resemblance. The 



Fig. 105. The structure of 

 the circulatory system just 

 after the blood begins to flow 

 in a complete circuit. The 

 vitelline veins have joined up 

 with the endocardium of the 

 heart. Aortic arches connect 

 the heart with the dorsal 

 aorta. The vitelline arteries 

 join the dorsal aorta. A very 

 complicated network of blood 

 vessels, which develops in the 

 area vasculosa, connects the 

 vitelline arteries with the vi- 

 telline veins. The flow of 

 blood is shown by arrows. 



