sm 



THE HUaLUf EMBRYO. 



organs, and more especially the fact that in it, as in the anterior 

 arches, an aortic arch, or branch of the trunens arteriosus, a', is 

 present, show that the ridge in question, be', is really a third 

 branchial arch. 



In Fig. 238 it is seen that the second branchial arches, 

 BR^, not only lie nearer the middle line than the first branchial 

 arches, be', but are also ia part overlapped by these. During 

 the latter part of the fourth week, this overlapping becomes 

 much more marked, the posterior visceral arches shifting for- 

 wards, and being telescoped witHn the arches in front of them. 



In Fig. 239 the condition at the end of the fourth week 

 is shown, at which time the first branchial arches have com- 



?"IG. 239. — The floor of the pharynx of a Human Embryo twenty-eight days 

 old, seen from above. Cf. Fig. 216, which represents the same embryo. 

 (From His.) x 30. 



A.3, third aortic arcli, in the first branchial arcli. A.4, f onrth aortic arcli, in the second 

 brancliiaL arcli. A.5, fifth aortic arch, in thCithird branchial arch. Bil.1, first branchial 

 arch. BB.2, second branchial arch. EB. membrane closing the hyomandibnlar cleft, 

 which afterwards becomes the tympanic membranei FK, foranien caecum. HY, 

 byoid arch. MN, mandibnlar arch. SU, sinus pnecervicalis. TH, median thyroid 

 rudiment. TU, tubercnlum impar. V.3. mandibular branch of trigeminal nerre. 

 Vn, hyoidean branch of facial nerve. IX, glosso-pharyngcal nerve. X, branchial 

 branches of pneumogastric nerve. 



pletely overlapped the second branchial arches, BE*, so as to 

 conceal them in surface views of the embryo. 



During the fifth week the first branchial arches are in their 

 turn overlapped and concealed by the hvoid arches (Fig. 240). 

 so that in surface views of embryos of this age none of the arches 

 behind the hyoid can be seen (ef. Fig. 205). 



