CH. LVIII.] 



DEVELOPMENT OF THE HEART. 



831 



Fig. 648 shows how the two primitive tubes have coalesced to 

 form one (H) beneath the anterior end of the alimentary canal, 



Fig. 646. Rabbit embryo of the 

 ninth day, seen from the 

 surface. (After Kcilliker.) 



Fig. 647. Diagrammatic section of embryo, 

 i'.epiblast; o,8omatopleur; Sp, gplanch- 

 nopleur ; P.P., the pleuro-peritoneal cavi- 

 ty between them ; H, heart ; N. C. , neural 

 cord ; If, notochord ; Pr., protovertebra ; 

 A.C., alimentary canal ; Y.S., yolk-sac. 



which is in this region quite cut off from the yolk-sac. On 

 each side of the notochord are seen two smaller tubes (A A) 



fig. 648. Diagrammatic transverse section of embryo. N, notochord ; A.C., alimentary 

 canal ; //, heart ; }'..S'., yolk-sac ; P.P., pleuro-peritoneal cavity. 



in section ; these are the transverse sections of the primitive 

 aortse. 



The heart so formed presents the appearance depicted in fig. 



