. Text-book of embryology. Embryology. Ill GILLS 169 Kespiratory lamellae develop only on its anterior wall and these, as development proceeds, become vestigial forming the pseudobranch. In Teleostean fishes the spiracular pouch (Fig. 89 A, vc. I) flattens out and disappears (Goette) so that the pseudobranch (ps.) on its anterior wall comes to lie on the inner face of the base of the operculum and appears to belong to the second cleft (Fig. 89, B). In Lung-fishes the solid endodermal rudiment never becomes perforate. It becomes gradually reduced during development while its outer ectoderma
. Text-book of embryology. Embryology. Ill GILLS 169 Kespiratory lamellae develop only on its anterior wall and these, as development proceeds, become vestigial forming the pseudobranch. In Teleostean fishes the spiracular pouch (Fig. 89 A, vc. I) flattens out and disappears (Goette) so that the pseudobranch (ps.) on its anterior wall comes to lie on the inner face of the base of the operculum and appears to belong to the second cleft (Fig. 89, B). In Lung-fishes the solid endodermal rudiment never becomes perforate. It becomes gradually reduced during development while its outer ectodermal portion becomes, as already indicated, converted into a special sense-organ. In Anurous Amphibians and in the Amniota the distal portion of the cleft rudiment becomes greatly dilated to form the tympanic cavity, while the proximal part forms the relatively narrow Eustachian tube. Just as the varying condition of the spiracle indicates a tendency for this cleft to undergo reduction so a similar but still more marked tendency exists for the gill clefts to become reduced at the other (posterior) end of the series. This is illustrated in the first place by the reduction in the number of functional clefts seen in passing from the lower Vertebrates to the higher. It is also frequently manifested in developmental stages. Thus amongst the Amphibia we find that in the Gymnophiona (Hypoge- ophis, Marcus) a rudimentary 7th cleft makes its appearance though it never reaches the ectoderm, while the 6th is open for a time. In Urodeles a 6th rudiment appears and is for a time connected with the ectoderm but does not become perforate, while in Anura this cleft appears only as a small and transient rudiment which never reaches the ectoderm. (III.) Internal Gills.—The internal gills or respiratory lamellae arise as ridge-like or, at first, finger-like projections of the cleft lining. The chief matter of dispute regarding their development has been the question whether they belong to the endo
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