TABANUS: THE PROVENTRICULUS 105 



staining well with eosin and faintly or not at all with haematoxylin, which 

 passes into all the fissures. (Plate XXI, fig. 4.) Internal to this there is 

 a single layer of columnar cells, which are very indistinctly separated 

 from one another, and are in many parts heaped together without any dis- 

 tinguishable cell membrane. The nuclei are oval or rounded, and lie 

 near the intima. The protoplasm is finely granular, and stains uni- 

 formly, though not well, with iron haematoxylin and similar stains ; 

 there is a narrow area just underneath the intima which stains more 

 deeply than the rest. External to these cells there is a rather thick base- 

 ment membrane. 



The muscular coat of the proventriculus is well developed, and con- 

 sists of a large number of coarse fibres scattered all over the surface ; 

 they are arranged in an irregular manner, and not in separate circular 

 and longitudinal bundles as is usually the case. 



The mamillation of the surface is due to the heaping up of cells 

 around the fissures, and it is especially in these elevated areas that the 

 cell membranes are difficult to define. Each elevation consists of a 

 mass of protoplasm with numerous scattered nuclei, with a central 

 area of finely granular eosinophile material, in the middle of which there 

 is a fissure which represents the lumen, and is in continuity with the 

 lumen of the rest of the tube. The expanded portions at the anterior end 

 are almost entirely made up of such little masses, and present a very 

 complex picture on section. The main portion of the lumen stops 

 short at the point where the oesophagus and the duct of the crop enter 

 the wall, and anterior to this there are only narrow fissures communi- 

 cating with one another and with the rest of the lumen. 



The relations of the oesophagus, the crop, and the proventriculus 

 are well brought out in a sagittal section. The lumen of the proven- 

 triculus appears as a narrow slit, with numerous fissures radiating 

 from it above and below ; the duct of the crop lies on the dorsal side 

 of the proventriculus, and appears as a direct continuation of the 

 oesophagus from the neck ; it communicates with the proventriculus by 

 an opening on the dorsal side of the latter, some distance from the 

 anterior end. 



At the posterior end the elevations become less marked, and the 

 tube is reduced to one very similar to the oesophagus ; it merges 

 gradually with the commencement of the mid-gut. 



The mid-gut is a large pear-shaped dilatation, with its narrowest end 

 anterior. When it is empty the wall has a mamillated appearance like 

 that of the proventriculus, but these markings disappear completely 

 14 



