POSTANAL SECTION OF ALIMENTARY CANAL. 449 



Its features at stage K are illustrated by an optical section 

 of the tail of an embryo (PI. 18, fig. 5) and by a series of trans- 

 verse sections through the tail of another embryo in PL 18, 

 figs. 6a, 6b, 6c, 6d. In the optical section there is seen a terminal 

 vesicle (alv^) opening into the neural canal, and connected with 

 the remainder of the alimentary tract. The terminal vesicle 

 causes the end of the tail to be dilated, as is shewn in PI. 8, 

 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 18, figs. 6a, 6b, 

 6c, and 6d} 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 epithelium, 

 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 extending through 

 about thirty sections (PL 18, fig. 6b}. Its average diameter is 

 about '084 Mm. Overlying its front end is the subnotochordal 

 rod (fig. 6b, 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 1 8, 6c, al.}. This for the most part has a fairly 

 uniform calibre, and a diameter of not more than "03 5 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. 6d\ 



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 



