Ill 



GILLS 



169 



A.r 



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 endodermal or 

 the ectodermal portion of the cleft lining. In cases where, as 

 frequently happens, the lamellae begin to develop after the cleft is 

 completely formed, the appearances are sometimes in favour of the 

 one sometimes in favour of the other interpretation. Goette (1901) 

 in fact goes the length of regarding the lamellae as being of endo- 

 dermal orio-in in the case of the spiracle and ectodermal in the case 



Fid. 89. — Horizontal sections through 

 Salmon embryos explaining position 

 of pseudobranch on inner surface of 

 operculum. (After Goette, 1901.) 



A.r, aortic root ; cm, aortic arch ; an, 

 anastomotic vessel connecting aortic arches 

 T and II ; By, hyoid arch ; op, operculum ; 

 Ph, cavity of pharynx ; ps, pseudobraDCh ; 

 vc, visceral cleft. 



