THE METAMORPHOSIS OF INSECTS 205 



The histogenetic reproduction of a tissue begins in the differentia- 

 tion and multiplication of small groups of cells, which were not affected 

 by the histolysis of the old tissue; such a group of cells is termed an 

 imaginal disc or a histoblast. They were termed imaginal discs on 

 account of the disc-like form of those that were first described and 

 because they are rudiments of organs that do not become functional 

 till the imago stage ; but the term histoblast is of more general appli- 

 cation and is to be preferred. 



The extent of the transformation of the internal organs differs 

 greatly in different insects. In the Coleoptera, the Lepidoptera, the 

 Hymenoptera, and the Diptera Nemocera, the mid-intestine and 

 some other larval organs are greatly modified, but there is no general 

 histolysis. On the other hand, in the Diptera Brachycera, there is a 

 general histolysis. In Musca all organs break down and are reformed 

 except the central nervous system, the heart, the reproductive organs, 

 and three pairs of thoracic muscles. Regarding the extent of the 

 transformations in the other orders where the metamorphosis is com- 

 plete we have, as yet, but little data. 



