426 THE ALIMENTARY CANAL OF THE LMAGO. 



of the original proctodseum becomes the rectum, which, accord- 

 ing to Schindler, always makes its appearance at a late period, 

 and which is entirely wanting in many larvae. 



Further, it is possible, and I think probable, that the 

 original proctodaeum is largely concerned in the formation 

 of the discs from which the generative ducts and cloaca of 

 the imago are subsequently developed. In the early stage on 

 which Biitschli [126, Taf. XXVI., Fig. 22, a, b, c] chiefly 

 relies for his evidence as to the origin of the Malpighian 

 vessels, it is more probable, I think, that he saw the rudiments 

 of the sexual organs than those of the Malpighian vessels, as 

 the union of the two sacs into a common tube, described by 

 him on page 542, is not consonant with what is known of the 

 development of the Malpighian vessels, nor with the fact that 

 they open by a pair of ducts into the alimentary canal. 



In all insects the anus is apparently developed late, and the 

 blind posterior extremity of the alimentary canal is certainly 

 not suggestive of the idea that the hind-gut originates from the 

 large primitive proctodaeal involution. 



The narrowing and lengthening of the chyle stomach, which 

 gives it its cylindrical form, occurs during the last five or six 

 hours of the embryo state, and the posterior part of the 

 intestinal tube grows rapidly in length during the same period. 

 The first flexure, which takes place as the result of the increase 

 in the length of the alimentary canal, is apparently the perma- 

 nent hctmal flexure between the Malpighian tubes and the 

 chyle stomach ; subsequently the intestinal coil is developed, 

 but this is proportionately very large in the embryo and newly- 

 hatched larva, so that it occupies nearly the whole body cavity. 

 These stages are figured by Weismann [2, Taf. VI., Figs. 79 

 and 80]. 



b. After-development of the Alimentary Canal. 



As has been already mentioned, the whole of the alimentary 

 canal of the larva, with the exception of the (esophagus and 

 the posterior extremity of the metenteron, becomes enclosed 

 within a fusiform sac, developed from parablastic elements 



