THE GERM-PLASM THEORY 399 



are formed during larval life, and these only enter on the stage of 

 formative activity after pupation, when they multiply rapidly and grow 

 together to form a segment, whose size, form, and external nature 

 is determined by them. But it is well known that the abdominal 

 segments of the fly differ from those of the larva very markedly 

 and in every respect, so that each cell-island must contain deter- 

 minants which are quite different from those in the skin-cells of 

 the corresponding larval segments. These last break up at the 

 beginning of pupahood, while the former begin to grow vigorously, 

 and to spread themselves out. The most remarkable fact about the 

 whole business, and it seems to me also the most instructive, is that 

 these imaginal disks frequently appear for the first time during larval 

 life, as I found in the case of a midge, Coretha lilwinicornis, in regard 

 to the disks of the thorax, and as Bruno Wahl ^ has recently demon- 

 strated in the case of the abdominal cell-islands. Since in the j'oung 

 larva the position of the subsequent imaginal disks is occupied bj' cells 

 which apparently in no way differ from the rest of the skin-cells, and 

 are also exposed to precisely the same external and internal influences, 

 the origination of the imaginal cells from these can only depend on 

 differential cell-division ; the primordial cell of each imaginal disk 

 must have separated at the beginning of disk-formation into a larval 

 and an imaginal skin-cell. 



In insects in which the larva and the imago differ widelj-, the 

 perfect insect, as regards all its principal parts, is already represented 

 in the larva, namely, in particular cells which lie among those of the 

 corresponding larval parts, and do not visibly differ from these, 

 although they are equipped with quite different determinants, and 

 consequently enter on their formative activity much later, and give 

 rise to quite different structures. As the determinants of the whole 

 animal with all its parts are contained in the ovum, so those of the 

 parts of its imaginal phase are contained in these cells of the imaginal 

 disks. 



In addition to all this, we have incontrovertible evidence in 

 favour of the theory of determinants in the independent phyletic 

 variations of the individual stages of development, on which depends 

 the whole phenomenon of ' metamorphosis ' which we have just been 

 considering. How could the larval stage have become so different 

 from the imago-stage, if the one were not alterable by variation 

 arising in the germ without the other being affected ? If this absolute 

 independence of the transmissible variability of the individual stages 



' Bruno Wahl, mber die EiUwickelung der hijpodeimalen Imaginalscheiben im Thorax und 

 Aldomm der Larve ron 'Eristalis' L., Zeitschr.f. wiss. Zool., Bd. Ixx. 1901. 



