THE DEVELOPMENT OF THE ,YKJ //'//. 343 



the nymph is a continuous cavity between the body wall and 

 the newly-formed alimentary canal, which contains the pseudo- 

 yelk. It soon, however, becomes permeated by a parablastic 

 network which binds together the alimentary sac and the em- 

 bryonic rudiments from which the internal organs of the imago 

 are developed. I believe this network is formed from the cells 

 which cover the extremities of the young tracheal vessels. 



The cell chaplets, which form the fringes of the dorsal vessel, 

 and some at least of the great pericardial cells of the larva, 

 remain and retain the same relative position as in the larva. 

 The pericardial fringes grow rapidly by the multiplication of 

 the cells of which they are formed. Their function and their 

 ultimate destiny are, however, unknown. 



Kowalevski [146] discovered that if the larvae are fed with flesh 

 impregnated with carmine, methyl blue, or silver salts, the peri- 

 cardial fringes, the great pericardial cells, and the cell chaplet 

 of Weismann become intensely coloured, and that the cells of 

 the pericardial fringes remain coloured, not only during the 

 whole pupa stage, but for some days after the imago emerges 

 from the pupa. These cells multiply so rapidly, according to 

 the above-named author, that they form a network which 

 clothes the whole of the posterior part of the mesophragma. 

 He also observes that the cell fringes of Weismann disappear 

 soon after the histolysis of the salivary glands is complete, and 

 that only seven pairs of pericardial cells remain in the imago. 

 The six posterior pairs degenerate. 



Flies fed with syrup coloured with cochineal, according to 

 Kowalevski, soon exhibit coloration of the pericardial cell 

 fringes. 



I have frequently found pericardial cells in the nymph on the 

 third day scattered amongst the constituents of the pseudo- 

 yelk of the posterior part of the abdomen. These undergo 

 histolysis like the fat bodies, and are probably the cells from the 

 posterior part of the pericardium. The peristent pericardial 

 cells are probably the elements from which the alar muscles of 

 the pericardium of the imago are developed, as I find them 

 embedded in the substance of their fibres. 



