ni BUCCAL CAVITY 151 



develops external to, and parallel with, the groove bounding the 

 primary tongue, and consequently lying on the floor of the mouth 

 between the primary tongue and the lower jaw. The thyroid 

 involution is situated between this thickening and the tip of the 

 tongue. 



The ectodermal thickening develops numerous glands, each 

 originating as a solid ectodermal down-growth, and is known as the 

 gland-field. Externally it is bounded by a shallow groove. Later 

 on the cleft or groove separating the gland-field from the primary 

 tongue becomes obliterated by fusion of its walls, and the gland-field 

 becomes raised up in a dorsal direction (Fig. 84, B) the tongue-tip 

 shrinking backwards so that eventually the demarcation between 

 primary tongue and gland-field disappears (Fig. 84, C). Meanwhile 

 the groove bounding the gland-field externally becomes deepened. 

 It forms the outer limit of the definitive tongue which is thus a 

 compound structure, its tip and edges developed from the original 

 gland-field, its postero-median part from the primary tongue. 



In the fishes the tongue remains non-muscular and non-glandular: 

 it is simply the primary tongue. In the Axolotl the tongue appears 

 also to be a primary tongue, the gland-field making a transient 

 appearance as a rudiment but eventually undergoing atrophy 

 (Kallius). 



In the Amniota the tongue is, as in the terrestrial Urodeles, a 

 compound structure, the primary tongue rudiment becoming fused 

 with an elevation of the floor of the mouth lying in front of the 

 Thyroid rudiment. This elevation, called by His the tuherculum 

 impar, represents morphologically the gland-field of the Urodeles. 



The tongue of Cyclostomes is remarkable for its complexity : it 

 has complex muscular and skeletal arrangements and on its surface 

 it develops the horny spines which function as teeth and simulate 

 teeth in their appearance. In Bdellostoma the tongue develops as a 

 cushion-like swelling of the floor of the mouth at an early period 

 while the velar membrane is still intact. In Petromyzon, on the 

 other hand, it does not develop until the time of metamorphosis. 



It has already been shown how the olfactory organs come to 

 communicate with the, buccal cavity by the posterior nares. In the 

 Amniota these become sunk into a recess in the roof of the mouth 

 ' and in the higher Beptiles, as in the Mammals, this recess becomes 

 shut off from the buccal cavity by a horizontal shelf which grows in 

 from the side and meets its fellow to form the palate. How this has 

 come about in evolution is illustrated by the three Lizards shown 

 in Fig. 85. 



In ontogeny the mode of origin may be similar, the palatine out- 

 growths meeting and fusing with one another in the middle line 

 (Crocodiles) or, as happens more usually, a median ridge or ■septum 

 extends backwards from between the primitive posterior naiWand 

 the palatine processes meet and fuse with its ventral edge. In the 

 two cases the physiological result is the same — the shunting back- 



