DEVELOPMENT OF THE BLOOD VESSELS IN THE FROG. 233 



form one continuous flap, which grows back as a kind of hood over 

 the sides and ventral surface of the throat, boxing in the gills. The 

 free posterior edge of the operculum soon fuses with the body along 

 the right side, about the level of the swelling caused by the head 

 kidney, a slight fold from the skin of the neck rising up to meet it ; 

 the fusion rapidly extends across the ventral surface towards the 

 left side, where the spout-like opening is formed, through which the 

 water is ejected from the opercular cavity in the act of expiration. 



Along the inner or pharyngeal borders of the branchial arches, 

 series of small transverse folds arise, which project into the cavity 

 of the pharynx and form the rudiments of the filtering processes, 

 which in later stages attain remarkable complexity (c/. Fig. 15, 

 F. 1, 2, 3, 4). 



Across the floor of the pharynx run two horizontal folds, which 

 meet each other posteriorly. These folds run from the sides of the 

 pharynx obliquely inwards and backwards, meeting in the median 

 plane just in front of the glottis ; their anterior edges are attached 

 to the floor of the pharynx, while their posterior edges project freely 

 backwards, overlapping the ventral ends of the gill arches ; the 

 purpose of these velar folds, as they may be called, appears to be 

 to ensure that the water taken into the mouth for respiration is 

 distributed equally to all the gill clefts. 



With regard to the gills ; the first branchial arch bears on the left 

 side a well-developed external gill, which commonly projects through 

 the opercular spout. On the right side the external gill is also 

 present, but is very small and shrunken. On both right and left 

 sides the first branchial arch bears along its ventral border a double 

 row of internal gill tufts, hanging down into the opercular cavity 

 (cf. Fig. 15, GL.1). 



The second branchial arch is very similar ; an external gill is 

 present on each side, that of the right side being much the smaller ; 

 a double row of branching gill tufts is borne along the ventral border 

 of each arch. 



The third branchial arch has a very small external gill, and also 

 a double row of internal gill tufts similar to those of the first and 

 second arches, but rather smaller. 



The fourth branchial arch has no external gill, but supports along 

 its ventral border a single row of internal gill tufts. 



