[FIG. 115. TRANSVERSE SECTION THROUGH THE DORSAL REGION OF AN EMBRYO CHICK OF 45 HOURS. _M.c._ medullary canal; _P.v._ mesoblastic somite; _W.d._ Wolffian duct; _So._ Somatopleure; _S.p._ Splanchnopleure; _p.p._ pleuroperitoneal cavity; _ao._ aorta; _v._ blood-vessels; _w._ germinal wall; _ch._ notochord; _op._ junction between area opaca and area pellucida.] In two succeeding pairs of visceral folds, which correspond with the hyoid and first branchial arches of the Ichthyopsida, are developed the parts of the hyoid bone, which will be best considered in connection with the development of the skull. The last two disappear in the Chick without giving rise to any permanent structures. The external opening of the first visceral _i.e._ hyomandibular cleft becomes closed[66], but the inner part of the cleft, opening into the mouth, gives rise to the Eustachian tube and the tympanic cavity, the latter being formed as a special diverticulum. [66] _Vide_ Moldenhauer, "Die Entwicklung des mittleren und des äusseren Ohres." _Morphologisches Jahrbuch_, Vol. III. 1877. Part of the membranous mandibular and hyoid arches form a wall round the dorsal part of the original opening of this cleft, and so give rise to the meatus auditorius externus. At the bottom of this is placed the tympanic membrane, which is probably derived from the tissue which grows over the dorsal part of the opening of the first cleft. It is formed of an external epiblast epithelium, a middle layer of mesoblast, and an internal hypoblastic epithelium. [FIG. 116. TRANSVERSE SECTION THROUGH THE TRUNK OF A DUCK EMBRYO WITH ABOUT TWENTY-FOUR MESOBLASTIC SOMITES. _am._ amnion; _so._ somatopleure; _sp._ splanchnopleure; _wd._ Wolffian duct; _st._ segmental tube; _ca.v._ cardinal vein; _ms._ muscle-plate; _sp.g._ spinal ganglion; _sp.c._ spinal cord; _ch._ notochord; _ao._ aorta; _hy._ hypoblast.] The general nature of the changes, which take place in the trunk between the commencement of the second half of the second day and the end of the third day, is illustrated by the sections figs. 115, 116, 117. [FIG. 117. SECTION THROUGH THE DORSAL REGION OF AN EMBRYO CHICK AT THE END OF THE THIRD DAY. _Am._ amnion; _m.p._ muscle-plate. _C.V._ cardinal vein. _Ao._ dorsal aorta. The section passes through the point where the dorsal aorta is just commencing to divide into two branches. _Ch._ notochord; _W.d._ Wolffian duct; _W.b._ commencing differentiation of the mesoblast cells to form the Wolffian body; _ep._ epiblast; _So._ somatopleure; _Sp._ splanchnopleure; _hy._ hypoblast. The section passes through the point where the digestive canal communicates with the yolk-sack, and is consequently still _open_ below.]