THE DEVELOPMENT OF THE NYMPH. 295 



head, etc. — of the Fly are in a far more advanced condition 

 within the larva than they really are; but the main point 

 remains, he recognised the fact that the parts of the nymph 

 are at lirst invaginated, and that their appearance on the 

 surface is due to evagination. And, further, he recognised a 

 distinct stage in the evolution of the nymph, when all these 

 parts are invaginated, and when the whole organism appears to 

 be little more than a simple sac, containing a fluid or semi- 

 fluid material ; I shall refer to this stage as the pronyiiibh. 



The observations of Reaumur quoted above agree very 

 closely with those of Van Rees, and with the following state- 

 ment which I made in 1872 : ' As yet a complete series of 

 investigations are wanting, but I have traced the steps of 

 development sufficiently to allow me to state that the great 

 procephalic lobes which exist in the half-developed embryo 

 become folded inwards, and lie one on either side of the 

 alimentary canal during the wdiole period of larval life. These 

 involuted procephalic lobes — and they are nothing else — form a 

 portion of the imaginal discs of Weismann, whilst the eyes, 

 antennae, and mouth organs are ultimately developed from 

 ceUular outgrowths at the bases of the same structures, just as 

 they are in the Crustacea ' [142]. The above is, I beheve, the 

 earliest notice of the invagination theory of the origin of the 

 imaginal discs, which I think more recent researches have 

 placed beyond a doubt. 



The development of the imago within the pupa-case will be 

 considered under the following heads : 



1. The formation of the pronymph from the larva. 



2. The development of the nymph from the pronymph ; and 



3. The development of the imago from the nymph. 



The hrst two form the subject of the present chapter, to 

 which a resiunc of the third has been added. The details of the 

 development of the various organs of the imago will, however, 

 be more conveniently elucidated in the second volume of this 

 work. 



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