156 Vertebrate Embryology 



mediate portion hanging as a loop between 

 them. The venous portion now forms a swell- 

 ing on each side, the rudiments of the auricles, 

 while the arterial end becomes enlarged to 

 form the beginning- of the bulbtis arteriosus. 



W.D.Seg.Sp.c 



Ec. Mes. 



Fig. 52. — Transverse section of a chick embryo with about 



TWENTY-EIGHT SEGMENTS. (After Minot.) 



C?^, caelom. jE'r, ectoderm. S"??^, entoderm. 7^/cJ', somatic mesoderm, mes,^ 

 Splanchnic mesoderm. A^, nephrostome. nek, notochord. See, segment. So7ny 

 somatopleure. Sp.c^ spinal chord. Spl, splanchnopleure. Ve, blood-vessel, WD., 

 Wolffian duct. 



The point of the loop will develop into the 

 ventricles (Fig. 57). 



The vascular system. — A single pair of aortic 

 arches has already been mentioned as extend- 

 ing from the bulbus arteriosus to the dorsal 

 side of the digestive tract, where each one is 

 continued to the posterior end of the embryo 

 as a dorsal aorta. Before the end of this day 

 the two dorsal aortse unite, behind the head, 



