ALIMENTARY CANAL. 



773 



of the alimentary tract by a very narrow thin-walled tube (C al}. This for 

 the most part has a fairly uniform calibre, and a diameter of not more than 

 035 mm. Its walls are formed of flattened epithelial cells. At a point not 

 far from the cloaca it becomes smaller, and its diameter falls to '03 mm. In 



FIG. 424. FOUR SECTIONS THROUGH THE POSTANAL PART OF THE TAIL 

 OF AN EMBRYO OF THE SAME AGE AS FIG. 28 F. 



A. is the posterior section. 



n c. neural canal; al. postanal gut; alv. caudal vesicle of postanal gut; x. 

 subnotochordal rod; nip. muscle-plate; ch. notochord; cl.al. cloaca; ao. aorta; 

 v.cait. caudal vein. 



front of this point it rapidly dilates again, and, after becoming fairly wide, 

 opens on the dorsal side of the cloacal section of the alimentary canal just 

 behind the anus (D al}. 



Very shortly after the stage to which the above figures belong, at a 

 point a little behind the anus, where the postanal section of the canal 

 was thinnest in the previous stage, it becomes solid, and a rupture here 

 occurs in it at a slightly later period. 



The atrophy of this part of the alimentary tract having once commenced 

 proceeds rapidly. The posterior part first becomes reduced to a small 

 rudiment near the end of the tail. There is no longer a terminal vesicle, 

 nor a neurenteric canal. The portion of the postanal section of the 

 alimentary tract, just behind the cloaca, is for a short time represented 

 by a small rudiment of the dilated part which at an earlier period opened 

 into the cloaca. 



In Teleostei the vesicle at the end of the tail, discovered by Kupffer, 



