552 



TIN-: HUMAN EMBRYO. 



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

 arches, an aortic arch, or branch of the truncus arteriosus, A 5 , is 

 present, show that the ridge in question, BR 3 , is really a third 

 branchial arch. 



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

 BR 2 , not only lie nearer the middle line than the first branchial 

 arches, BR 1 , but are also in 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 within 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- 



FlG. 239. — The floor of the pharynx of a Human Embryo twenty-eight clays 

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

 (From His.) x 30. 



A.3, third aortic arch, in the first branchial arch. A.4, fourth aortic arch, in the second 

 branchial arch. A.5, fifth aortic arch, in thethird branchial arch. BR.1, first branchial 

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

 which afterwards becomes the tympanic membrane. FK, foramen caecum. HY,' 

 byoid arch. MN, mandibular arch. STJ, sinus prascervicalis. TH, median thyroid' 

 rudiment. TTJ, tuberculum irnpar. V.3- mandibular branch of trigeminal nerve. 

 VII, hyoidean branch of facial nerve. IX, glosso-pharyngeal nerve. X, branchial 

 branches of pneumogastric nerve. 



pletely overlapped the second branchial arches, br 2 , 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 hyoid arches (Fig. 210), 

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

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



