278 



STUDY OF PIG EMBRYOS. 



The dorsal root of the ganglion joining the spinal cord, Sp.c, is shown on both 

 sides of the section, and the nerve itself also appears, being best shown on the 

 left side of the embryo, where a short piece, R.D.-$, of the ramus dorsalis is 

 included and a much longer piece, ^.^.3, of the ramus ventralis. Just inside of 

 the nerve at the level of the notochord, Nch, is the cross-section of the vertebral 

 artery. On the right side of the embryo the section passes through a portion o.f 



Sp.c. 



FIG. 195. PIG, I3--.0 MM. TRANSVERSE SERIES 5, SECTION 353. 



Ao, Aorta. Ao.4, Fourth aortic a>vh. Au.d, Right auricle. Au.s, Left auricle. Card, Anterior cardinal vein- 

 Cos, Coelom. d.IV, Four*" gill-pouch. .3, Ganglion of third cervical nerve, msth, Mesothelium. N . 

 ctrv.2, Second cervical nerve. Nch, Notochord. N.IO,II, United vagus and spinal accessory nerves. P. A, 

 Pulmonary artery. PA, Pharynx. R.D. 3, Dorsal ramus of the third cervical nerve. R-V.$, Ventral ramus 

 ' cervical nerve. Som, Somatopleure. Sp.c, Spinal cord. Sym, Sympathetic nerve chain. Tra, 

 Trachea. Ve, Vein to lower jaw. X 22 diams. 



the second cervical nerve, N.cerv.2. The anterior cardinal, Card, is a very large 

 vessel. Close to its ventral wall appear a few fibers which represent the first cer- 

 vical rterve, but they are too indistinct to be represented 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 to form the main aortic arch of the body, 



