INTESTINAL CANAL. 



383 



organ from distinct imaginal discs take place, as with the hypo- 

 dermis, side by side, in such a way that the continuity is not 

 anywhere interrupted. "We owe our knowledge of the imaginal 

 discs of the intestinal canal to Ganin (No. 107). More recently, 

 Kowalevsky (No. 112) and Van Kees (No. 121) have described the 

 development of the intestinal canal in detail. 



The imaginal discs of the intestine, which in the pupa is very short, 

 are found in the enteron 



in the form of numer- HI j- 



ous scattered cell-islands 

 (Fig. 189, ie); in both 

 the stomodaeum and 

 proctodaeum these ap- 

 pear as a ring (v and h) 

 of imaginal tissue capable 

 of great increase. The 

 imaginal ring of the 

 fore-gut (v) lies in the 

 region of the so-called 

 proventriculus (pr, cf. 

 Fig. 191, im), while 

 that of the hind -gut 

 is to be sought immedi- 

 ately below the aperture 

 of the Malpighian ves- 

 sels. 



m -^. 



— rru 



The regeneration 



Fig. 190.— Transverse section through the pupal mid-gut 

 of one of the Muscidac (after Kowalevskv). e, degen- 

 erating larval epithelium ; /, the newly-formed cell- 

 layer round the same ; m, muscular coat ; m', imaginal 

 cells of the muscular coat; o, imaginal discs of the 

 mid-gut epithelium ; t, tracheal trunks. 



of these two parts of 



the intestinal canal is not exclusively brought about by these two 

 rings, but the imaginal rudiments of the neighbouring parts of the 

 body- surface participate in it. It thus appears that the most 

 anterior part of the oesophagus is yielded by the imaginal discs 

 round the mouth, while the discs of the eighth abdominal segment 

 that surround the anus (Fig. 189, ims) produce, by invaginating, 

 the rectal sac and the rectal papillae. 



The development of the permanent mid-gut (chylific stomach) 

 proceeds in such a way that the island-like imaginal discs, increasing 

 considerably in number, extend over the external or basal surface 

 of the epithelium of the larval enteron (Fig. 190, o) until they come 

 into contact and fuse, the wall of the imaginal intestine being thus 

 formed. The whole of the larval enteric epithelium (e) is at the 

 same time cast off into the interior of the gut and, surrounded by 



