440 



the rest of the foregut does not occur till a series of remark- 

 able processes have taken place in the anterior half of the 

 midgut; only then does the foregut extend backwards, and, 

 occupying the place of the anterior half of the true (endo- 

 dermal) midguts of the larva and early pupa, fuses with the 

 posterior half, which remains as the stomach, being all that 

 survives of the old midgut. The midgut will, therefore, most 

 conveniently be considered first. 



(B) The Metamorphosis of the Midgut and the Development 

 of the Post-oesophayeal part of the Foregut. 



I have already described the midgut as composed of a 

 single layer of large flattened cells, with smaller cells at their 

 bases. As many of these smaller cells do not s urvive in the 

 V, imago, I shall refer to them here as replacing cells. 



Neither larval nor replacing cells, so far as I can observe, 



(proliferate during larval life. The larval cells grow enormously 



in size, and, through the pressure exerted upon them by the 



( contents of the midgut, are seen, at the end of the third day, 



1 as great flat cells, with smaller replacing cells at their bases. 



V But during the third day these replacing cells begin slowly 



to divide by mitosis, while the great larval cells remain 



inactive. 



But at the end of the fourth day of larval life a great 

 change begins. At about this time the rectal ingrowth at 

 last fuses with the midgut; and this event is marked by the 

 commencement of a series of contractions of the muscles of 

 the intestine which gradually drives the undigested food, 

 which has accumulated here during larval life, to the exterior. 

 This is the defaecation period of the larva, and lasts from 

 one to two hours. But it is apparently under the pressure 

 exerted by the muscles that a remarkable process of -dis- 

 integration of the epithelium of the larval midgut begins. 

 The disappearance of the faecal material allows the cells to 

 return to the cubical conditions in which they existed in the 

 new-born larva. 



In the cytoplasm of the midgut epithelial cells before 

 defaecation, vacuoles were already becoming numerous; 

 usually, however, it was quite granular and showed obvious 

 signs of degeneration. Not only had the cells grown greatly, 

 but the nuclei had greatly hypertrophied, and were to be 

 seen as long, irregular, faintly granular, chromatin masses, 

 devoid of nucleoli. When the cells contract these long nuclei 

 become bent; and a section presents the curious but false 

 appearance of large multinucleated cells. 



But at defaecation the vacuolation becomes very much 

 more marked (fig. 144) ; the vacuoles consist perhaps of fatty 



r 



