VARIATION AND MUTATION 143 



between the congenital and acquired condition is at hand, 

 thanks to the unusual character of the development of certain 

 specialized insects, namely, all those that undergo a complex 

 metamorphosis. 



"Without by any means exhausting the subject of the postembry- 

 onic development of insects, entomologists have become sufficiently 

 well acquainted with the phenomena attending this development to be 

 able to confirm absolutely (in essential characters) Weismann's dis- 

 coveries in the larva of the ' imaginal discs ' as the independent embry- 

 onic centers from which develop the wings, legs, antennae, and some 

 other parts of the winged adults (imagines) of insects with complete 

 metamorphosis. That is to say, in all the insects which hatch from the 

 egg in a larval condition markedly different from the definitive condi- 

 tion of the species in its fully developed, mature stage, many of the 

 adult organs, as the external parts of the head, and the legs and wings, 

 are produced not by a gradual development, growth, and transforma- 

 tion of the corresponding larval parts, but by a special development in 

 late larval life and during the pupal stage, the final structures being 

 formed from small groups of previously undifferentiated subembryonic 

 cells. These cells are derived in the case of the external parts just 

 named chiefly from i-nvaginations of the larval cellular skin layer. In 

 the larva (maggot) of a house fly, for example, there are no functional 

 legs or wings: there are no external signs (buds, pads) of these organs at 

 any time in the larval stage. 



"In the larval life there can be no possible molding influence on 

 these future adult organs of the nature of a direct response or reaction 

 to the immediate environment. We might assume such an influence 

 possible if the wings and legs were slowly transforming external struc- 

 tures subject to attempts at or actual functional use in flight or crawling 

 during the larval life. At pupation, the wings and legs suddenly 

 appear as external parts, but still equally functionless, and now wholly 

 concealed and protected by the opaque chitinized wall of the puparium. 

 With the final issue of the adult, the wings and legs appear for the first 

 time in functional condition, and with the simple need of unfolding, 

 expanding, and drying the outer wall, an operation requiring but few 

 moments, they appear at this time in their definitive fully developed 

 condition. The wings have the arrangement of veins and number of 

 spines and fringing hairs; the legs have the armature of spines and 

 spurs and the number of segments which they retain unchanged through 

 the short or longer adult life. The wings and legs of the adult of all 



