EMBRYO OF 6 MM. STUDIED IN SECTIONS. 



247 



into young muscle-fibers. In the 12 mm. pig in this region the cells of the muscle- 

 plate have already broken apart and no distinct plate can any longer be recognized. 

 Below the muscle-plate follows the section of the anterior cardinal vein, Card. 

 Lower down and in the median line we have the section of the pharynx, Ph, 



Ven.IV. 



Card 



A .car- 



V en. 111. Md. 



FIG. 181. PIG, 6.0 MM. TRANSVERSE SERIES 9, SECTION 90. 



A.car, Carotid artery. Card, Card', Anterior cardinal vein. Md, Medullary wall of the fore-brain. A'.i, X.2 

 JV.3, N.4, Neuromeres of the hind-brain. A*. 5, Trigeminal ganglion. N.-j, 8, Acustico-facial ganglion. 

 N.g, Root of the glosso-pharyngeal nerve. Ot, Otocyst. Ven.III, Third ventricle or cavity of the fore-brain. 

 Ven.IV, Fourth ventricle. X 35 diams. 



lined by the epithelial entoderm. The pharynx is surrounded by the very large 

 aortic vessels, which start from the ventral side of the pharynx, and pass upward 

 along its sides to join the descending aorta, Ao.d.^, at about the level of the jugu- 

 lar veins. The vessels shown are the fourth aortic arches. Their symmetry and 



