POST-EMBRYONIC DEVELOPMENT OF CULEX. 179 
constriction to the beginning of the stomach, elongates 
and becomes slightly folded. From its hinder part, close 
to its opening into the stomach, a blind sac grows out 
ventrally and rapidly extends back under the stomach to 
the hinder part of the thorax, and numerous sacculations 
develop along its sides. This is the ‘‘ crop”’ of the imago. 
No trace of it is to be seen in the youngest pupe. The 
stomach is sharply marked off from the cesophagus by a 
constriction, and in the young pupa by the character of its 
epithelial cells (see fig. 7). During pupal life the epithelium 
splits into two layers (see figs. 3 and 4) and the inner layer, 
by far the thicker, undergoes disintegration and is diges- 
ted (fig. 5), so that at the end of pupal lie hardly a trace 
of it is left (fig. 6). In the intestine the epithelium is 
longitudinally folded and the change is more difficult to 
make out. Here also the splitting of the epithelium and 
disintegratioa of its inner layer occur. 
In the rectum more complex changes occur, and the 
tube, which was at first of almost uniform diameter 
throughout, becomes differentiated into a very wide an- 
terior portion, the rectal pouch, and a narrow posterior 
portion the rectum proper. The wall of the rectal pouch 
at the same time becomes as it were pushed in at four 
points so as to project in the form of tour papillee into the 
cavity. The epithelium of these rectal papille (or ‘‘ rectal 
elands’’) becomes highly specialised, its cells being very 
large and columnar. Mesoblastic tissues with trachez 
and nerves extend into the axis of each. Throughout both 
portions of the rectum the epithelium splits and the inner 
layer is shed. This layer is thick except over the papille 
where it is very thin and difficult to recognise, while the 
basal layer, which forms the permanent epithelium of the 
rectum is, except on the papillee, so thin that it is difficult 
to see, 
