THE G2SOPHAGUS, STOMACHS AND INTESTINES. 421 
stantly bathed by water renewed by the pulsatile action of the 
rectal pouch ; but, as Chun [164] observes, they cannot have a. 
respiratory function in the fly, in which they are usually closely 
surrounded by a mass of excrement. Treviranus [161] and 
Newport regarded them as glands, an opinion which I held in 
1879 [163], and which I still maintain as correct. 
This view has not been accepted by the majority of authors. 
Neither Leydig nor Chun so regard them, yet, if the folding of 
the wall of the rectum were reversed, no one would for a 
moment have doubted their glandular character. 
In my former paper on these glands I suggested that they 
are renal organs. In the pupa stage they secrete insoluble 
urates, which give the murexide test with great ease, but in 
the adult fly their secretion is fluid. The excrement gives off 
ammoniacal fumes when heated, and fails to give the murexide 
test. 
Rhythmic Pulsations.—During life the rectal papilla pulsate 
rhythmically. These pulsations can be observed in the female 
through the transparent skin of the ovipositor when the latter 
is exserted, as these organs then lie within it. The pulsations 
are due to the contraction of the radiating muscles and the 
elasticity of the outer capsule; they evidently serve to insure 
the influx and efflux of blood to and from the central cavity. 
4, DEVELOPMENT OF THE ALIMENTARY CANAL. 
a. In the Embryo. 
The origin of the stomodeum, mesenteron, and proctodzeum 
of the embryo has already been discussed, in relation with that 
of the hypoblast. There is a perfect concord amongst ob- 
servers as to the origin of the alimentary canal from three dis- 
tinct sources. The stomodeum and proctodeum are derived 
from the epiblast, and the mesenteron from the hypoblast ; 
and numerous observations, my own included, indicate the 
correctness of this view. 
It is not difficult to demonstrate the limits of the stomo- 
