441 



material and occasionally contain feebly staining grains. 

 They are suspended in a very faintly granular "spongioplasm." 

 As the pressure exerted by the muscles increases the cells 

 begin to project irregularly into the intestinal lumen; and 

 then a very remarkable thing is to be observed. The spongio- 

 plasm, together at times with the vacuoles, begins to ooze out 

 through the cell membrane, and hangs as one or more large 

 drops, suspended in the intestine from the degenerating cells 

 (fig. 144). The process commences in the anterior part of the 

 midgut , but soon extends right along it, as the faecal con- 

 tents are gradually voided. The products of degeneration are 

 themselves, however, retained in the lumen of the intestine. 

 There they granulate and are seen sometimes as little balls 

 of grains, at other times as a fine dust. Eventually the whole 

 of the cytoplasm is cast into the now very contracted lumen 

 of the midgut;' and all that remains is the cell membrane 

 containing a very degenerate looking clump of chromatin 

 grains. But these are soon added to the mass of debris which 

 now consists of fine grains, of small clusters of grains, of 

 fragments of nuclei, and of the contracted walls of the dead 

 cells; all forming a dark mass that now occupies the lumen 

 of the intestine. In the larva eight hours after defaecation 

 these changes are complete, and all the old larval epithelium 

 has disappeared, with the exception of a narrow strip of 

 dead cells running along either side of the intestine from one 

 end to the other (fig. 146). The temporary retention of these 

 cells is an extraordinary adaptation for bringing about the 

 destruction of the hepatic caeca; their fate will be described 

 later. 



It is necessary to return now to the replacing cells. In 

 the defaecating larva these cells have begun to proliferate by 

 mitosis, and by the time the larval cells have lost most of 

 their cytoplasm (i.e., about four hours after defaecation), 

 these have formed a completely new epithelium, closely sur- 

 rounding the degenerate larval epithelium. This gives the 

 intestine the false appearance of having a functional two- 

 layered epithelium (fig. 145). But as the larval epithelium 

 disappears more and more, the cells o*f the new epithelium 

 increase in size, and in the eight-hour pupa alone persist, 

 except for the two thick bands of dead cells on either side 

 of the intestine, close beside the great hepatic caeca. 



But shortly after this an extraordinary thing is to be 

 observed. The cells of the renovated epithelium, growing 

 in size, begin to push the two longitudinal columns of dead 

 cells into the lumen of the intestine. To each of these columns 

 — the sole remains of the epithelium of the larval midgut — 

 the hepatic caeca, which have now grown 70/x in thickness, 



N 



