445 



the archenteron ; usually, in other insects, this occurs during 

 embryonic life. 



This event is quickly followed by muscular contractions in 

 the midgut, and two hours later the whole of the faecal 

 matter, which has accumulated during the three days of 

 active feeding, is voided. The rectal musculature takes part 

 in the process; only a small part of the faecal matter at a 

 time is passed into the rectum, and this, under the pressure 

 of the muscular walls, is rounded off into a little pellet, which 

 is forced slowly along the rectum. 



Meanwhile the epithelial cells have begun to degenerate. 

 The nuclei are large and granular, the karyosomes having 

 scattered their material through the nucleoplasm as this 

 gradually hypertrophied. The cytoplasm then undergoes 

 granular degeneration. In the defaecating larva these 

 granules cluster together in little balls and breaking through 

 the cell membrane float about in the blood stream, where they 

 may become engulfed by phagocytes; or, if these are not 

 present at the time, simply dissolve in the blood plasma. At 

 other times the cytoplasm becomes broken up into a number 

 of larger hyaline globules, like those of the integumental cells. 

 They share the same fate as do the balls of granules. 



Meanwhile, the cells of the rectal imaginal ring grow 

 backwards as well as forwards, and as the rectal epithelium 

 disintegrates they rapidly replace it. Already in the defae- 

 cating larva the epithelium of the anterior quarter of the 

 rectum is composed entirely of embryonic cells, and these, 

 dividing mitotically, are actively growing backwards, replacing 

 the epithelial cells as these disintegrate more and more. A 

 few hours later the whole larval epithelium has disappeared, 

 and a loose layer of spindle-shaped embryonic cells has re- 

 placed it. The muscle layer does not disappear till a few 

 hours after pupation. 



The cells of the renovated epithelium now consolidate 

 their position. In the larva some eight hours before pupa- 

 tion a cuticle is being secreted between the old cuticle and the 

 epithelium, and when the larva moults the old cuticle of the 

 last larval instar is drawn out through the anus. 



In the fresh pupa myoblast cells, which were present in 

 only small numbers during larval life, having proliferated 

 considerably during the last twenty-four hours, now form a 

 distinct layer outside the rectum, and it is not till several 

 hours later that the old larval muscles begin to disintegrate 

 ajid- become phagocjtised. 



The "renovated hmdgut epithelium now begins to differ- 

 entiate into two regions; the small spacious rectum behind 

 and the small intestine in front. 



