426 THE ALIMENTARY CANAL OF THE IMAGO. 
of the original proctodzum becomes the rectum, which, accord- 
ing to Schindler, always makes its appearance at a late period, 
and which is entirely wanting in many larve. 
Further, it is possible, and I think probable, that the 
original proctodzum 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 proctodzal 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 hemal 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 8o]. 
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 oesophagus and 
the posterior extremity of the metenteron, becomes enclosed 
within a fusiform sac, developed from parablastic elements 
