218 



STUDY OF PIG EMBRYOS. 



the very large aortic vessels, which start from the ventral side of the plmrynx 

 and pass upward along its sides to join the descending aorta, Ao. d. 4, at about 

 the level of the jugular veins. The vessels shown are the fourth aortic arches. 



D.z. 



///,iv. 



Fig. 129. — Pig, 9.0 mm. Transverse Series 9, Section 171. 

 Ao.d.4, Descending aorta receiving the right fourth aortic arch. cl.III, Third entodermal gill cleft. D.Z, 

 Dorsal zone of spinal cord. F.B, Fore-brain. Hy, Hyoid branchial arch. Jug, Jugular vein. Mdb, 

 Mandibular branchial arch. My, Muscle plate. N, Nerve. Nek, Notochord. Olf, Olfactory plate. Ph, 

 Pharynx. ^[Z, Ventral zone of spinal cord. /,//,///, /F, First to fourth ectodermal gill clefts. X 35 

 diams. 



Their symmetry and their relations to the pharynx are beautifully demon- 

 strated in this section. Below the aorta we find a section of the third internal 



