ALIMENTARY CANAL. 



635 



B 



postanal gut begins to develop a terminal dilatation or vesicle, connected 

 with the remainder of the canal by a narrower stalk. 



The walls both of the vesicle and stalk are formed of a fairly columnar 

 epithelium. The vesicle communicates in front by a uai-row passage with 

 the neural canal, and behind is continued into two horns corresponding 

 with the two caudal swellings previously spoken of (p. 45). Where the 

 canal is continued into these two horns, its walls lose their distinctness 

 of outline, and become continuous with the adjacent uiesoblast. 



In the succeeding stages, as the tail grows longer and longer, the post- 

 anal section of the alimentary tract grows with it, without however under- 

 going alteration in any of its essential characters. At the period of the 

 maximum development, it has a length of about \ of that of the whole 

 alimentary tract. 



Its features at a stage shortly before the external gills have become 

 prominent are illustrated by a series of transverse sections through the 

 tail (fig. 424). The four sec- 

 tions have been selected for 

 illustration out of a fairly- 

 complete series of about one 

 hundred and twenty. 



Posteriorly (A) there is 

 present a terminal vesicle 

 (ale) '25 mm. in diameter, 

 which communicates dorsally 

 by a narrow opening with 

 the neural canal (w) ; to 

 this is attached a stalk in 

 the form of a tube, also lined 

 by columnar epithelium, and 

 extending through about 

 thirty sections (B al). Its 

 average diameter is about 

 084 mm., and its walls are 

 very thick. Overlying its 

 front end is the subnoto- 

 chordal rod (x), but this does 

 not extend as far back as the 

 terminal vesicle. 



The thick-walled stalk of 

 the vesicle is connected with 

 the cloacal section of the ali- 

 mentary tract by a very nar- 

 row 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 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 



al- 



FIG. -i2i. FOUR SECTIONS THROUGH THE POST- 

 ANAL PART OF THE TAIL OF AN EMBRYO OF THE SAME 

 AGE AS FIG. 28 F. 



A is the posterior section. 



uc. neural canal; al. post-anal gut; air. caudul 

 vesicle of post-anal gut ; .r. subnotochord rod ; mp. 

 muscle-plate ; cli. uotochord ; cl. al. cloaca ; ao. 

 aorta; v. can. caudal vein. 



