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 arteriosits. 



Som. W.D.Seg.Sp.c. 



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, coelom. EC, ectoderm. Ent, entoderm. Mes, somatic mesoderm. mes* 

 splanchnic mesoderm. JV, nephrostome. nek, 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 aortae unite, behind the head, 



