74 THE LARVA OF THE BLOW- FLY. 



which possesses such inherent improbability, that even if there 

 were no observations in support of more probable views, one 

 would have been tempted to doubt the conclusions on which it 

 rests, even although the facts observed are substantially as 

 stated by Weismann and Ganin. 



The most rudimentary discs (Fig. 3, /) do not appear very 

 distinctly until the pupa stage ; they are mere groups of em- 

 bryonic cells, which project on the surface of the hypodermis, 

 and are the rudiments of the abdominal somites. In some 

 sections of larvae I have detected small groups of embryonic 

 cells in the same position ; and, from the characters of the discs, 

 I think it probable that they are derived directly from epiblast 

 cells. 



Ganin says ' the transformation of the great polygonal cells 

 of the larval hypoderm into small embryonic cells commences 

 at four points in the lateral regions of the segments, and the 

 new formation is preceded by a separation of the hypoderm 

 and muscles of the larva and its cuticle.' He adds, ' these 

 cells arise from the old cells of the larval hypoderm.' These 

 extracts are quoted by Viallanes [27, p. 215], who remarks 

 that, ' The replacement of the larval by the imaginal hypo- 

 dermis is analogous to that of the milk by the permanent 

 teeth in mammals. In the insect four germs exist in each 

 somite, which at a certain period undergo evolution, and de- 

 termine the shedding of the hypodermis of the larva.' 



Viallanes appears to me, in this and the following statements, 

 to have correctly appreciated the true morphological character 

 of the imaginal discs. He says : 



' The facts observed by Weismann in Corethra, and those 

 discovered by Klinckel in Volucella, must lead us to believe 

 that the discs are derived from the hypoderm [or epiblast].* 

 In insects we [frequently] observe that the imaginal discs are 

 united with the hypoderm, either directly, or by a more or less 

 elongated pedicel. The variations which are observed in 

 different species, or in the same species in different parts, 



* The words in brackets are added by me, and, with their addition, I 

 entirely agree with Viallanes. 



