FORMATION OF BLOOD VESSELS AND THE HEART 

 FOLDS OF AMNION 



ECTODERM 



175 



SOMATIC MESODERM 



CHORION 



AMNION 



SPLANCHNIC MESODERM 

 FUSION 



AMNIOTIC CAVITY 



COELOM i 



SOMATOPLEURE 



YOLK STALK 



ENDODERM SPLANCHNOPLEURE 



Fig. 103. The lateral folds of the amnion and the closure of the gut. The 

 ectoderm and somatic mesoderm form lateral folds. They meet and fuse over the 

 top of the embryo and thus enclose an amniotic cavity. The outer double layer 

 is the chorion ; the inner is the amnion. The amnion extends ventrally, and the 

 gut is constricted into a tube which communicates with the yolk sac through the 

 yolk stalk. A double layer of somatic mesoderm and ectoderm is called somato- 

 pleure. A double layer of endoderm and splanchnic mesoderm is known as 

 splanchnopleure. 



Formation of blood vessels 

 and the heart 



A most dramatic event in development is the process of heart formation. 

 By a simple, direct fusion of two tubes a functional heart is formed in an 

 interval of some twelve hours of development. At the same time, the anterior 

 end of the primitive digestive tract is laid down. 



As the early blastoderm spreads over the surface of the yolk, blood islands 



