THE. (ESOPHAGUS, STOMACHS AJVD 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 papillae 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 stomodaeum, mesenteron, and proctodaeum 

 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 stomodaeum and proctodaeum 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- 



