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 bulbus arteriosus. 



Som. W.D.Seg.Sp.c. N EC. Mes. 



Coe. 



Ve. nch. Ent. Spl. mes. 1 



FIG. 52. TRANSVERSE SECTION OF A CHICK EMBRYO WITH ABOUT 



TWENTY-EIGHT SEGMENTS. (After Minot.) 



Coe, ccelom. EC, ectoderm. Ent, entoderm. Mes, somatic mesoderm. mes,* 

 splanchnic mesoderm. N, nephrostome. nch, notochord. Seg, segment. Som, 

 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, 



