THE DIGEST I VE TRACT. 



Some time before the displacement of the inner ves- 

 icle is completed, and when the folds which wil] cut off the 

 peribranchial cavity are not very deep, the wall of the inner 

 vesicle high up on the left at the extreme posterior end, be- 

 comes much thickened, and soon evaginates to produce a little 

 blind pouch, the rudiment of the digestive tract. Fig. 24, d_. 

 tr. This lateral diverticulum grows out as a tube, which at 

 once bends sharply downwards and for.vards, v/hile, as the 

 shifting of the vesicle continues, its opening into the lat- 

 ter is carried up nearer and nearer the mid-dorsal line, 

 where it will ultimately come to lie. The tube soon turns 

 abruptly on itself to form a close U, and nov; grov/ing up- 

 wards along the outer wall of the left peribranchial sac un- 

 til it reaches the dorsal surface, ^i^t- finally bends directly 

 forward and stops short at the posterior v/all of the cloacal 

 cavity. At this point the distal extremity of thetube fuses 

 with the cloacal wall, an opening breaks through, and the 

 anus is established. The differentiation into oesophagus, 

 stomach and intestine takes place very early, and is apparent 

 at a stage considerably younger than that shown in Fig. 4. 

 The course and development of tne tube are sufficiently il- 

 lustrated by Pigs. 4 and 5. 



As the bud grov;s and increases in length, the di- 

 gestive tract enlarges enormously, the U becomes opened more 

 and more, and the intestine describes a v;ide curve which lies 



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