422 THE ALIMENTARY CANAL OF THE IMAGO. 
dum, since it remains a comparatively simple tube, and 
exhibits at all times in the life of the insect the characters of 
an epiblastic structure ; a flat pavement epithelium and a dis- 
tinct chitinous intima. It is true that it is somewhat difficult 
to define the precise limit of the stomodzum in the proventri- 
culus, and to determine how far the development of this organ 
is from epiblastic and how far from hypoblastic cells; but if 
we adopt the evidence afforded by the character of the epithe- 
lium, it is clearly chiefly developed from the hypoblast of the 
mesenteron—a view which is rendered probable by the manner 
in which the development of the proventriculus takes place. 
It is quite another matter, however, to determine the fate of 
the proctodzal involution. Many observers, including myself, 
have seen a distinct proctodzal involution in the early stages 
of embryonic life. In the Blow-fly embryo, however, Graber 
has evidently regarded my blastopore as an anus and my 
metenteron as a proctodeum. If the section from which my 
figure (Fig. 2) is taken is to be interpreted as I have inter- 
preted it, there is clearly a proctodzum, which is quite distinct 
from the dorsal invagination; but I have been quite unable to 
determine its fate. 
I have carefully searched for evidence that the hind-gut is 
formed from the proctodzal involution; and I am not aware 
that anyone has been more successful than myself in actually 
tracing the development of the hind-gut (metenteron) to the 
proctodeum. Biitschli’s figures [126, Taf. XXVII.] are far from 
convincing as to the real origin of the Malpighian tubes, espe- 
cially Fig. 31, b, in which these tubes appear to arise from the 
mid-gut ; and his figures of a still earlier stage are capable of 
another interpretation (see p. 426). 
The frequently repeated text-book statements that the Mal- 
pighian vessels originate from the anterior extremity of the 
proctodzum rest upon indirect, rather than direct, evidence. 
Although Swammerdam figured the alimentary canal of 
the Bee larva [4, Pl. XXIV., Fig. 6], and represented it 
as a continuous alimentary tract, it has been more recently 
described as consisting of a proctodzeum which is blind at its 
