248 



STUDY OF PIG EMBRYOS. 



their relations to the pharynx are beautifully demonstrated in this section. Below 

 the aorta we find a section of the third internal gill-cleft, cl.III, a narrow, slit- 

 like cavity lined by entoderm. By following the series of sections, the connection 

 of this cavity with that of the pharynx can be traced, thus demonstrating that the 



Olf. 



F.B 



D.Z. 



Nch. 



Ill, IV. 



FIG. 182. PIG, 6.0 MM. TRANSVERSE SERIES 9, SECTION 171. 



A.d.4, Descending aorta receiving the right fourth aortic arch. Card, Anterior cardinal vein. cl.III, Third 

 entode.rmal gill-cleft. D.Z, Dorsal zone of spinal cord. F.B, Fore-brain. Hy, Hyoid branchial arch. Mdb, 

 Mandibular branchial arch. My, Muscle-plate. N, Nerve. Nch, Notochord. Olf, Olfactory plate. 

 Ph, Pharynx. V.Z, Ventral zone of spinal cord. I, II, III, IV, First to fourth ectodermal gill-clefts. 

 X 35 diams. 



cleft is an evagination of the pharynx, as are all the gill-clefts. On the left-hand 

 side of the embryo the junction of the entoderm of the internal pouch with the 

 ectoderm is shown. The two germ-layers have united to form a typical closing 

 plate. Above the third gill-cleft, the outline of the embryo shows a deep depres- 



