160 THE FROG. 



manner with, the external epiblast, yet does not open to the 

 exterior. Shortly before the mouth opening is established, the 

 hyomandibular gill-pouch separates from the external epiblast 

 and recedes somewhat from the surface. The two hypoblastic 

 lamellaB separate from each other, so as to form a saccular 

 diverticulum from the pharynx, and this gradually opens out 

 into the cavity of the pharynx, and in tadpoles of about 20 mm. 

 length ceases to be recognisable as a distinct pouch. 



The Eustachian tube and tympanic cavity are formed near 

 to the hyomandibular pouch, but independently of it, and in a 

 manner which has already been described in the section dealing 

 with the development of the ear (p. 143). 



2. The Gills. 



There are two sets of gills in the tadpole, external and 

 internal respectively ; the former being branching processes pro- 

 jecting outwards from the first three branchial arches on each 

 side, while the internal gills are formed later as vascular tufts 

 on the sides of all four branchial arches. The two sets of gills 

 differ in some important respects, and it is generally considered 

 that they are independent series of structures. 



The external gills appear shortly before the time of hatching, 

 as two pairs of small, backwardly directed processes from the first 

 and second branchial arches. They are at first somewhat conical 

 in shape, with rounded or very slightly notched borders : the 

 gill of the first arch overlaps that of the second arch, and is 

 placed rather more ventrally than this latter. 



By the time of hatching (Figs. 72 and 73, BR.I, BR.2), the 

 external gills have increased in size. The first one is notched 

 at its free posterior border into three blunt lobes ; and the 

 second into two or three similar ones. 



In the succeeding stages the external gills grow rapidly, and 

 the lobes into which they are divided become larger and more 

 numerous. A third external gill appears on the third branchial 

 arch of each side (Figs. 73, 74) : it is very small, and is overlapped 

 and almost concealed by the two anterior gills. 



The external gills attain their maximum development about 

 the time of opening of the mouth. At this stage (Figs. 44, 5, 

 and 77), they form much-branched plumose tufts, exceeding 



