THE DEVELOPMENT OF THE NYMPH. 337 



appears that these muscles become surrounded by a vast num- 

 ber of cells, parablast or mesenchyme, and that this mass of 

 cells persists and forms the nidus in which the wing muscles 

 appear. There is nothing to show that one particle of the 

 larval muscle remains. I am far from denying the facts alleged 

 by Van Rees, but there is no question that the wing muscles 

 are developed from mesoderm cells which grow from the meso- 

 thoracic disc, and that these grow into a mass of parablast. 

 That this cellular parablast consists of leucocytes, which ac- 

 cumulate around certain dorsal muscles of the larva, is possible ; 

 but the larval muscle fibres described by Van Rees as the last 

 to degenerate in the thorax form no part of the new muscles. 



The dorsales and sterno-dorsales are at first minute and 

 slender strings of cells imbedded in an abundant parablast. 

 They first appear on the third day of the pupa state as six, 

 not three, bands of cell tissue, chiefly parablastic in origin, of 

 which nothing remains in the end except the nuclei, with, per- 

 haps, some remnant of their protoplasm giving off long stellate 

 processes, which form a connective reticulum for the support 

 of the new muscle fibres. The development of the dorsales 

 and sterno-dorsales in Chironomus can be distinctly watched 

 in the living animal, and it is clear that, although developed 

 between the fibres of the larval muscles, the latter take no part 

 in their formation. I must hold, therefore, that the develop- 

 ment of the muscles of the imago is not from those of the larva, 

 although it seems likely that the vast numbers of leucocytes which 

 surround these muscles may form coherent parablastic tissue 

 into which the mesoderm cells of the discs grow. It may even 

 happen, in the case of the wing muscles, that fragments of 

 the larval muscles sometimes remain longer than in that of 

 other muscles, although I have not observed the fact ; but the 

 new muscles neither originate from the nuclei of the old ones, 

 nor in their substance by the transformation of existing muscle 

 tissue. Neither do they arise from the leucocytes which sur- 

 round the larval muscles. They are developed in every case 

 from cells which grow from the discs themselves, and the para- 

 blast which surrounds these cells forms at most a connective 



