THE ANATOMY OF TEN TO TWELVE MM. PIG EMBRYOS 



129 



Sept.H 



bulb into aorta and pulmonary trunk, the left joins the interventricular septum, 

 while the right extends to the endocardial cushion. These folds eventually fuse 

 and the partition of the ventricular portion of the heart is completed. The en- 

 docardium at the atrio-ventricular openings is already undermined to form the 

 anlages of the tricuspid and bicuspid valves. From the caudal wall of the left 

 atrium is given off a single pulmonary vein. 



The Arteries. — As seen in Fig. 118, the first two aortic arches have dis- 

 appeared. Cranial to the third arch, the ventral aorta? become the external 

 carotids. The third aortic arches and the cephalic portions of the descending 

 aorta; constitute the internal carotid arteries. The ventral aortx* between the 

 third and fourth aortic arches persist as the common carotid arteries. The de- 

 scending aortae in the same region are 

 slender and eventually atrophy. The 

 fourth aortic arch is largest and on the 

 left side will form the aortic arch of 

 the adult. From the right fourth arch 

 caudad, the right descending aorta is 

 smaller than the left. Opposite the 

 eighth segment, the two aortae unite 

 and continue caudally as the median 

 dorsal aorta. The fifth aortic arches 

 (the sixth of human embryos) are 

 connected with the pulmonary trunk, 

 and from them arise small pulmonary 

 arteries to the lungs. Dorsal interseg- 

 mental arteries arise, six pairs from the 



descending aorta;, others from the dorsal aorta. From the seventh pair, which 

 arise just where the descending aortas fuse, the subclavian arteries pass off to 

 the upper limb buds and the vertebral arteries to the head. The latter are 

 formed by a longitudinal anastomosis between the first seven pairs of interseg- 

 mental arteries on each side, after which the stems of the first six pairs atrophy. 



Ventro-lateral arteries from the dorsal aorta supply the mesonephros and 

 genital ridge (Fig. 118). Ventral arteries form the cceliac artery to the stomach 

 region, the vitelline or superior mesenteric artery to the small intestine, and the 

 inferior mesenteric artery to the large intestine. 



The umbilical arteries now arise laterally from secondary trunks which 

 persist as the common iliac arteries. 

 9 



Fig. i2o.- 



-Heart of 12 mm. embryo dissected 

 from the right side. 



