22 A.VA TOMY AND MORPHOLOG Y OF INSECTS. 



Weismann supposed that only the integument (hypoderm) of 

 the nymph is developed from the discs, but it is now certain 

 that they consist of both epiblast and mesoblast, and that all 

 the tissues of the nymph are re-developed from these and 

 similar rudiments. 



Nomenclature of the Embryonic Rudiments of the Nymph. — The 

 term ' imaginal disc,' although very appropriate for the struc- 

 tures from which the head, thorax, and abdomen, with their 

 appendages, are developed, is less suited for the designation of 

 the rudiments of the viscera and nervous system. Kiinckel 

 d'Herculais* proposed to substitute the term ' histoblast,' but 

 this met with no general acceptance ; it may, however, be 

 useful as a general term for those rudiments which are destined 



Fig. 3. — A Semi-diagrammatic Representation of the Imaginal Discs, i, a simple 

 disc ; 2 anil 3, leg discs ; 4 and 5, wing discs ; c, cuticular layer of skin ; 

 //, hypoderm cells ; s, sac of disc ; d, disc. 



to form the viscera (splanchnoblasts) and nervous system 

 (the neuroblast). 



Histolysis. — It will be seen that there is no marked line of 

 demarcation between the Ametabola and the highest Metabola, 

 so far as the origin of the organs of the imago is concerned ; 

 but the two groups are distinguished by other phenomena — 

 those of histolysis, or the degeneration of the larval tissue — of 

 the highest significance, which separate the most specialised 

 Insecta, with a rapid metamorphosis, from the less specialised, 



* ' Recherches sur TOrganisation et Je Ddveloppement des Volucelles.' 

 Fol., Paris, 1875-81. 



