The Development of the AlHgator 2']'] 



about the same appearance as in the preceding 

 figure, though the former seems somewhat thinner. 

 Figure 12c is just back of the bent-under fore- 

 brain represented in the preceding figure and in 

 front of the main body of the heart. The plane of 

 the section not being at right angles to the long 

 axis of the body (as was mentioned above), the 

 figure is not bilaterally symmetrical. The neural 

 canal, since the section passes through the auditory 

 vesicles, may here be called the hindbrain {]ih). It 

 has an almond-shaped cavity, surrounded by a wall 

 of medium thickness. In close contact with the 

 wall of the hindbrain, on each side, is the inner 

 side of the auditory vesicle {p), which is seen as a 

 deep, wide-mouthed pit in the superficial ectoderm. 

 On the right side of the section the auditory pit 

 is cut through its middle region; it is simply a 

 thickened and condensed area of the ectoderm 

 which has been invaginated in the usual way. 

 Directly beneath the hindbrain is the notochord 

 (w/), on each side of which, in the mesoblast, is 

 the dorsal aorta {ao) , or rather the continuation of 

 the aorta into the head. Beneath these structures 

 and extending from one side of the section to the 

 other is the pharynx {p}i) ; its lining wall is fused 

 on each side with the ectoderm, but there is no 

 actual opening to the exterior. These points of 

 contact {£) between entoderm and ectoderm are of 

 course the gill clefts; they are not yet visible from 

 the outside. The roof of the pharynx is flat and 



