FORMATION OF THE PRO NYMPH FROM THE LARVA. 311 



Barfurth's idea at first sight has apparently more to recom- 

 mend it, but there is no evidence that the muscles attacked are 

 either feeble or dying, although they are no longer functionally 

 active ; and the metenteron of the larva, although functionally 

 inactive, resists the attacks of leucocytes perhaps longer than 

 any other larval tissue, and appears to be scarcely altered even 

 on the third and fourth days of the pupa stage. 



The theory that a chemical ferment exists in those tissues 

 which are about to be removed and renders them capable of 

 being invaded by leucocytes is equally unsatisfactory, as such 

 ferments are soluble and could scarcely fail to infect the fluids 

 and the body. I regard the subject as one on which specula- 

 tions are useless. 



g. Histolysis and other Changes in the Alimentary Canal. 



The changes of the alimentary canal in the first days of the 

 pupa state have been investigated b}- Ganin, Kowalevski, and 

 Van Rees, and, although some discrepancies exist in the 

 accounts they give, the following facts may be said to have 

 been established. 



The continuity of the cavities of the stomodaeum and mesen- 

 teron is broken, and the posterior part of the oesophageal tube 

 becomes an impervious cord. The crop is first contracted 

 and then drawn into the oesophagus, and the cuticular intima 

 of both the crop and oesophagus is shed with the pharyn- 

 geal armature of the larva. The muscular and cellular coats 

 of the oesophagus and crop are subsequently removed by 

 histolysis. The pharyngeal skeleton of the larva is withdrawn 

 from the cephalic cleft in the pronymph, and a new stomodaeum 

 is developed. 



Weismann [2] observed the gross changes which occur in 

 the remainder of the alimentary canal. He says : ' Already 

 on the second day the chyle stomach, w^hich in the larva 

 is more than 1-5 centimetres long, has become shortened 

 to about -6 centimetres, and the openings of the Malpighian 

 tubules, the boundary between the chyle stomach and intestine, 

 are drawn forwards.' But, as Weismann shows in his figure, 



21 



