الفهرس 
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Abstract
(3) The Bralchial Arches And Neck : The construction of jaws and a neck is closely bound up wi th the history of the brEl1.ChiaL arches. These are bar- like ridges, separated by grooves, which appear on each ventrolateral surface of the embryonic head during the fourth week. • The human embryo develops five such arches, separated by four ectodermal branchial grooves. At the same levels as these external grooves the entoderm of the pharynx pushes aside the mesenchYme and bulges outward to become the phary- ~eal pouches. The ectoderm of each groove and the entoderm of its complementary pouch then meet and unite. As a result, a typical arch is separated from the one ahead and behind it by a thin epithlial plate only. The first branchial arch on each side bifurcates into a maxillary and a mandibular p roce ase s a’I’he last arch lies caudal to the fourth cleft and is poorly defined along its caudal margin (Arey, 1965). During the sixth week the second arch overlaps the next three and obscures them, the more caudal arches then sinking into a triangular depression called the cervical sinus~long wi th this the ectodermal grooves become drawn out into branchial ducts. At least that part of the sinus