[Branchial arch development in the @entity35 and @entity19 . II. The existence of XXXX ].

@entity19 @entity2419 @entity2085 @entity35
The development of the branchial arch region was examined at 6- to 12-hour intervals in @entity19 embryos at the age of 9.5-11.5 days after conception [middle of @entity2085 ( @entity2085 ) 10 to middle of @entity2085 12] and in @entity35 embryos at the age of 10.5-12.5 days after conception (middle of @entity2085 11 to middle of @entity2085 13) using SEM, serial semithin sections and wax plate reconstructions. The 4th branchial groove is always separated from the corresponding pharyngeal pouch by a broad layer of mesenchyme. At no time did we find a membrana obturans in this region. However, during development of the mesenchyme disappears between the branchial grooves and pharyngeal pouches of the first four arches, allowing external ectodermal epithelium to come into contact with the internal endodermal epithelium to form a membrana obturans. In its dorsal parts this membrane is formed by two layers of epithelium, in the ventral portion frequently it consists of only one layer of epithelium. It was not possible to determine whether the latter was derived from ectoderm or endoderm. Degeneration of cells in this ventral part of the membrane leads to openings which we consider to correspond to @entity2419 . The possibility of an artefactual genesis of these membrane ruptures is discussed. The "1st @entity2419 ' appears in @entity19 embryos 15 the age of 9.75 days (second of @entity2085 10), the "2nd @entity2419 ' appears first at the age of 10.5 days (middle of @entity2085 11), and the "3rd @entity2419 ' appears at the age of 11 days (end of @entity2085 11). In the @entity35 , the corresponding developmental stages are seen about 24 h later. Whereas the "lst and 3rd @entity2419 ' are demonstrable during a period of maximally 12 h, the "2nd @entity2419 ' is present during the developmental period of about 24-36 h. This protracted existence of the "2nd @entity2419 ' is possibly related to the occurrence of lateral cervical fistulas and cysts in the region of the second branchial arch.
