REGENERATION OF DIGESTIVE EPITHELIUM 127 



be seen still retaining the impress of the depressed villi against 

 which they were formed. At a later stage the whole of the pigment 

 is collected in the middle of the posterior part of the chamber, being 

 driven there by the repeated contractions of the wall. * 



As the blood pigment increases in amount it is passed through 

 the sphincter muscle into the hind-gut, where it mingles with the 



secretion of the Malpighian tubes. According to 



. 5 , , ... . 6 f Contents of hind-gut 



Schaudinn, the quantity of the latter, which is of a 



yellowish white colour, is increased as absorption of food from the 

 mid-gut proceeds, and one should, therefore, be able to estimate the 

 stage of digestion which exists in the gut from the colour of the 

 faeces. The problem is complicated, however, by the fact that the hind-gut, 

 like the mid-gut, is in a state of active peristalsis during digestion, 

 as is shown by the frequently made observation that blood-sucking 

 flies defaecate while feeding. The faeces first passed during the act 

 of feeding consist of dark tarry-looking masses, which may be either 

 the remains of the last meal, as is most probable, or freshly-formed 

 pigment. Later fresh red blood is passed in many cases. In caught 

 flies one almost always finds some granules of pigment in the hind-gut, 

 in the form of small hard crystalline masses. 



As the contents of the gut are absorbed the cells lose their flattened 

 shape ; they become first cubical, and later assume their ordinary columnar 



form as the wall contracts around the diminishing 



.... , . , , i , Regeneration of cells 



volume of blood. Those which have degenerated and 



passed out into the lumen are replaced by the growth of the small cells 

 which are always to be found between the regular columnar cells, and 

 at their bases, in the condition of the rest. These increase in size 

 within a few hours of the feed. In one to three days, according to the 

 species, the temperature, and the size of the meal ingested, the gut 

 assumes its normal appearance, and globules destined to be discharged 

 at the next meal accumulate within the cells. When, as in Tabanus, 

 the cells are heaped up into villi, these appear at an early stage, while 

 there is still plenty of blood in the gut. 



In the foregoing no account is given of the chemical aspects of the 

 digestive process. The amount of information on this question is 

 at present neither large enough nor sufficiently exact to be of 

 practical use. 



* The corpuscles tend to accumulate at the posterior end of the mid-gut, and the serum 

 at the anterior end, as a result of this peristalsis. 



