TRANSVERSE SECTIONS OF EMBRYO OF 12 MM. 



281 



tral side of the trachea and somewhat toward the left, but quite close to it, ap- 

 pear two small blood-vessels, the pulmonary arteries. They are so small that their 

 lumina scarcely show in the figure. Two sections nearer the head, the two ves- 

 sels are found united in a single stem. Opposite the arteries below the trachea 



Sp.c. 



D.R. 



Nch 



F. 



Som. 



FIG. 196. PIG, 12.0 MM. TRANSVERSE SERIES 5, SECTION 470. 



Ao.S, Left descending aorta. Au.d, Right auricle. Br.Plx, Brachial plexus. C.C.S, -Vena cardinalis communis 

 sinislra. D.R, Dorsal root of spinal nerve. F, Cardiac fissure. G, Spinal gangfion. L, Anterior limb-bud 

 Nch, Notochord. Nv, Branch of brachial plexus. (E, (Esophagus. R.D, Ramus dorsalis of spinal nerve. 

 S.a.c, Septum of the auricular canal. ScLV, Subclavian vein. Som, Somatopleure. Sp.c, Spinal cord. 

 S.s, Septum superius. Tra, Trachea. Val, Auriculo-ventricular valve. Ven.S, Left ventricle of the heart 

 V.R, Ventral root of spinal nerve. X 22 diams. 



a minute opening, not shown in the figure, marks the tip of th 

 chamber. To the right and left of the oesophagus appear th 

 of the two descending aorta-, of which the left, Ao.S, is alread' 

 the right. Ultimately the greater part of the right / 



iwal 



