220 POSTANAL SECTION OF ALIMENTARY CANAL. 



nected with the remainder of the alimentary tract. The ter- 

 minal vesicle causes the end of the tail to be dilated, as is 

 shewn in PI. vil. fig. K. The length of the postanal section 

 extending from the abdominal paired fins to the end of 

 the tail (equal to rather less than one-third of the whole 

 length of the embryo), may be gathered from the same figure. 



The most accurate method of studying this part of the 

 alimentary canal is by means of transverse sections. Four 

 sections have been selected for illustration (PL xvii. fig. 6a, 6Z>, 

 6c, and Qd) out of a fairly-complete series of about one hundred 

 and twenty. 



Posteriorly (fig. 6a) there is present a terminal vesicle 

 •25 Mm. in diameter, and therefore rather smaller than in 

 the earlier stage, whose walls are formed of columnar epi- 

 thelium, and which communicates dorsally by a narrow opening 

 with the neural canal; to this is attached a stalk in the 

 form of a tube, also lined by columnar epithelium, and ex- 

 tending through about thirty sections (PL xvii. fig. 66). Its 

 average diameter is about '084 Mm. Overlying its front end 

 is the subnotochordal rod (fig. 66, 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 alimentary tract by a very narrow thin- 

 walled tube (PL xvii. ^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 a flattened epithelium. 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 

 (fig. U). 



Near the close of stage K at a point shortly behind the 

 anus, where the postanal section of the canal was thinnest in 

 the early part of the stage, the alimentary canal becomes solid 

 (PL XI. fig. ^d), and a rupture here occurs in it at a slightly 

 later period. 



In stage L the posterior part of the postanal section of the 

 canal is represented by a small rudiment near the end of the 

 tail. The rudiment no longer has a terminal vesicle, '}ior does 



