VIII 



AETHEOPODA 



269 



and intestine. The cells on the stomodaeal side elongate so as to 

 block the cavity of the oesophagus : they cast out refractive granules, 

 and then, at the final moult, flatten out and form the lining of the 

 adult oesophagus. The cells on the mid-gut side of the valve multiply 

 very rapidly and form a plug completely obliterating the lumen of the 

 mid-gut. The old mid-gut epithelium is stripped off from its base- 

 ment membrane and is devoured by amoebocytes penetrating from the 

 body-cavity. The whole mid-gut is shortened till it is about one 

 fourth of its former length, but Poyarkoff could not determine how 

 this comes about. Then a new provisional epithelium is reconstituted 



stem 



B 



FIG. 213. Portions of sections through the valve separating stomoilaeum from mid-gut 

 iu the larva of <iah-ni<-cUn_ xl/iti. (After Poyarkolf. ) 



A, in the well-developed larva. B. in the larva on the point of metamorphosis, rut, cuticle lining 

 the stomodaeum ; l.i-iid, larval endoderm ; ii.eml, pupal endoderm ; stom, stomodaeuni ; v, valve. 



from some of the cells forming the plug. This epithelium lasts 

 during the pupal stage, at the close of which it is thrown off and 

 the definitive epithelium of the mid-gut is formed from other cells 

 of the plug which have hitherto remained undifferentiated. 



The epithelium lining the hind-gut or proctodaeum behaves very 

 much like that lining the stomodaeum ; the cells cast out granules 

 and then flatten, in other words the basal part of each cell rejuvenates 

 itself. Then the folds in the proctodaeum flatten out and the cells 

 become cylindrical ; this shape they retain in the pupal stage, but 

 when the adult stage is reached these cells flatten out again. The 

 salivary glands of the larva are entirely destroyed by amoebocytes, 

 but new salivary glands are formed in the pupa when it is three days 



