10 BUTTERFLIES 



years had passed was it definitely replaced by another and 

 much more convincing theory propounded by Weismann. 

 According to Weismann's theory, which is now well- 

 estabhshed, the process of development internally is a 

 much more continuous one than the external changes 

 would indicate. So far as the latter are concerned we 

 simply say that a caterpillar changes to a chrysalis and a 

 chrysalis to a butterfly, the transition in each case requir- 

 ing but a very short time. Internally, however, it has 

 been going on almost continuously from the early life of 

 the caterpillar. The various organs of the butterfly arise 

 from certain germinal disks or "imaginal" buds, the word 

 "imaginal" in this case being an adjective form of imago, 

 so that the imaginal buds are really simply buds for the 

 starting of growth of the various organs of the imago or 

 adult. As the caterpillars approach the chrysalis period 

 these imaginal buds rapidly develop into the various organs 

 of the butterfly. This process is helped along by the 

 breaking down of many of the tissues of the larva, this 

 broken-down tissue being then utilized for the production 

 of the new organs. About the time the chrysalis is formed 

 this breaking-down process becomes very general, so that 

 the newly formed chrysalis seems largely a mass of creamy 

 material which is soon used to build up the various parts 

 of the butterfly through the growth of the imaginal buds. 



The Change to the Butterfly 



{See plate, pages 32-33.) 



There is probably no phenomenon in the world of living 

 creatures which has attracted more attention than the 

 change of the chrysahs into the butterfly. It is not 

 strange that this is so. We see upon a tree or shrub or wall 



