194 



STUDY OF PIG EMBRYOS. 



which represent the first cervical nerve, but they are too indistinct to be repre- 

 sented in the figure. They may easily be found with the higher power. In the 

 median plane is the crescent-shaped section of the pharynx, Ph. Between the 

 jugular vein and the pharynx lies the fourth aortic arch, Ao. ^. The right and 

 left arches are at this stage about equal in size, although the left arch is destined 



Au.S. 



Fig. 119. — Pig, 12.0 MM. Transverse Series 5, Section 353. 

 Ao, Aorta. A0.4, Fourth aortic arch. Au.d, Right auricle. Au.S, Left auricle. Cce, Coelom. cl./V, 

 Fourth gill pouch. G.j, Ganglion of third cervical nerve. Jug, Jugular vein, msth, Mesotheliura. 

 N.cerv.2, Second cervical nerve. Nch, Notochord. N. 10,11, United vagus and spinal accessory nerves. 

 P. A,, Pulmonary artery. Ph, Pharynx. R.D. 3, Dorsal ramus of the third cervical nerve. R.V.j, 

 Ventral ramus, of the third cervical nerve. Som, Somatopleure. Sp.c, Spinal cord. Sym, Sympathetic 

 nerve chain. Tra, Trachea. Ve, Vein to lower jaw. X 22 diams. 



to form the main aortic arch of the body, and only a portion of the right arch 

 will persist to form a portion of the stem of the pulmonary artery. The figure 

 indicates the manner in which these aortic arches pass up from the heart below 

 on either side of the pharynx. A little above the aortic arch on either side may 

 be seen a small, round spot, Sym, which is somewhat conspicuous on account of 



