772 POSTANAL SECTION OF THE MESENTERON. 



following the dilated portion which gives rise to the branchial cavity 



and permanent intestine. It has already 



been shewn that from the dorsal and 



lateral portions of this section of the 



primitive alimentary tract the notochord 



and muscles of the Ascidian tadpole are 



derived. The remaining part of its walls 



forms a solid cord of cells (fig. 423, al'}, 



which either atrophies, or, according to 



Kowalevsky, gives rise to blood-vessels. 



In Amphioxus the postanal gut, FIG. 423. TRANSVERSE OPTICAL 



.hough distinctly developed, is no, very % 

 long, and atrophies at a comparatively (After Kowalevsky.) 

 early period. The sect i on ; s f rom an embryo of 



In Elasmobranchii this section of the the same age as fig. 8 iv. 



alimentary tract is very well developed, ch - notochord ; nc neural 1 canal ; 



. , , me. mesoblast ; of. hypoblast of 

 and persists for a considerable period of ta ji < 



embryonic life. The following is a 

 history of its development in the genus Scyllium. 



Shortly after the stage when the anus has become marked out by the 

 alimentary tract sending down a papilliform process towards the skin, the 

 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 narrow passage with 

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

 with the two caudal swellings previously spoken of (p. 55). Where the 

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

 outline, and become continuous with the adjacent mesoblast. 



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 -J 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 sections 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 (alv) '25 mm. in 

 diameter, which communicates dorsally by a narrow opening with the 

 neural canal (nc) ; 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 subnotochordal 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 



