154 EMBRYOLOGY OF THE LOWER VEETEBBATES < EL 



ir.c 



lamellae or internal gills, respiratory organs of another type the 

 external gills. As there is some reason to believe that these are m< <iv 

 ancient organs than the gill-clefts they will here be considered first 

 although they are much less familiar than the clefts with their internal 

 irills. The branchial organs will therefore be considered in the follow- 

 ing order : (I.) External gills, (II.) Visceral clefts, (HI.) Internal ^ills. 

 (I.) EXTERNAL GILLS. The true external gills are organs which 

 are commonly confounded with the ordinary or internal gills developed 

 in the walls of the gill-clefts. They appear however to be quite in- 

 dependent of these in their origin and they would probably have 

 attracted more attention and interest than they have done had it not 

 been for the fact that they occur in their typical form in only three 

 subdivisions of the Vertebrates (Crossopterygii, Dipnoi, Amphibia) 

 and that two out of these three groups comprise animals of extreme 



rarity, the developmental 



v.a.IV stages of which have not 



e.g.1 ^^ been generally accessible 



to embryologists. 



The typical External 

 gill is a projection from 

 the surface of the body 

 on the outer side of a 

 visceral arch. It con- 

 sists of a core of mesen- 

 chyme with a covering 

 of ectoderm ; it is tra- 

 versed by a vascular loop 

 consisting of the main 

 aortic arch which passes 

 out to its tip and then 



doubles back ; and it commonly has a pinnate form, paired projections 

 growing out so as to increase its respiratory surface. It is provided 

 with muscles by means of which the possessor is able to flick it 

 sharply backwards so as to renew the water in contact with it. 



The external gill as a rule is without any special skeletal support 



bui in the larval Polypterus a short rod of cartilage projects into its 



base, and in t he extinct Dolichosoma of the Gas coal of Bohemia there 



pparently present a well-developed segmented skeleton within 



the substance of the external gills. 



The external gill develops as an outgrowth from the tissue of tin- 

 visceral arch at a period at which the clefts are not yet perforated. 

 It arises as a luil^iim of the surface (Fig. 86) and in the author's 

 opinion the endoderm of tin- cleft, rudiments takes no part in its 

 formation. At the same time it is only right to state that the pre- 

 valent, opinion in the past ha> l>e,-n different. The outer surface of 

 the viscer.il arch m the region when- the external ^ill will de\elop is 

 covered by a la yer of cells thicker than the neighbouring ectoderm, 

 and in some cases this thickened portion of the ectoderm sho\\s in its 



end. 



FlG. 86. Diagrammatic longitudinal section through the 

 early rudiments of the external gills of Gt 

 (Stage 25). 



e.g, external trill ; end, emloderm ; v.a, visceral arch : 

 visceral cleft rudiment. 



