438 



But shortly after feeding ceases the epithelial cells begin 

 to degenerate. The nuclei are granular and greatly hyper- 

 trophied, and possess a large nucleolus, so characteristic of 

 the degenerating cells of Nasonia. In the defaecating larva 

 the cytoplasm undergoes granular degeneration ; or, at other 

 times, it breaks up into larger globules, which, breaking from 

 the cells, float about in the blood and are there engulfed 

 by any leucocytes which happen to be present, or, if left to 

 themselves, dissolve in the blood. 



Tliese changes are accompanied by an active regeneration 

 of the foregut (fig. 117). During larval life (though 1 can- 

 not say definitely at which period of it) a proliferation of the 

 cells of the oesophageal imaginal ring has occurred, and 

 these, forsaking their ordinary cubical shape, elongate and 

 become spindle-shaped (fig. 152). Continuing to divide mito- 

 tically they bulge outwards, and at the same time extend 

 forwards, and in the defaecating larva are to be seen actively 

 replacing the disintegrating larval cells. Although I did 

 not observe them penetrating these cells, as in the case of the 

 myoblasts extending over dead muscles, yet it seems probable 

 that they actively absorb the products of disintegration and 

 grow at their expense, so near do they lie to the dead larval 

 cells. 



The oesophageal epithelium is partly regenerated also 

 from another centre, viz., the integumentary imaginal discs of 

 the first segment; and in the defaecating larva these embryonic 

 cells are to -be seen extending through the mouth inwards, 

 between, or over the dead and disintegrating cells of the larval 

 epithelium, while the proliferating cells of the oesophageal 

 ring extend forwards to meet them (fig. 117). About four 

 hours after defaecation the two have met. 



Meanwhile there has begun a proliferation of certain 

 myoblast cells, which lie during the whole larval life scat- 

 tered about in the head in the neighbourhood of the mouth ; 

 these extend backwards as a loose column of very long 

 spindle-shaped cells, drawn out in long thread-like processes 

 at either end (fig. 117). They form the musculature of the 

 anteriof" part of the oesophagus. Others are to be seen behind 

 the oesophagus; the muscles which they form are differently 

 disposed from those of the anterior side of the oesophagus, and 

 will be considered later. The development of the great 

 pharyngeal dilator muscles is considered in connection with 

 the development of the general muscular system. 



During the remainder of larval life the cells, occasionally 

 still dividing, settle down, and growing in size, co-operate to 

 form a single epithelium — that of the adult oesophagus. This 

 development is accompanied by a great bending downwards 



