THE GERM-PLASM THEORY 



pressure previously exerted on the cell by it. surround! 

 material cause of a structure is to be sought for uol in th, di- 



turns of its appearance, but in the primary conuti ate which have 



been handed on to the relevant cell or cell-group from the germ in 



other words, through its determinants. I | 11W . ,„,. ;„.,.,, ,:„,,., „„. 



blunt rounded knob of the rough and clumsily jointed i cells 



which represents the insect's leg at the beginning of the pupal period 

 (tig. 93 ,A) be incited to thicken, to const,!,., ;ll the rool 

 to form a joint-surface, to broaden out at tl ,,.[. and pi 



Fig. 93. The development of a limb in the pupa of ;i Fly 

 camaria). A, apex of the limb from a pupa four days ■ .Id ; the jointing i- hinl 

 at ; hy, hypodermis ; ps, pupal sheath ; ph, phagocytes ; V. trachea] branch. 

 the same on the fifth day; the lumen of the limb i^ quite ailed with phagocj 

 (p/a); the last tarsal joint (£ 5 ) is beginning t<> -di"\v a I >i lid ;i|> . , the same 

 the seventh day ; the claws (Kr) and the adhesive lobes hi are formi d. 



two sharply cut points (C), which become incurved and form claws 

 (Jcr), while beneath these a broad flat lobe (A/) grows forward, and 

 with its regularly disposed cells gradually forms the characteristic 

 adhesive organ of the fly — how could all tin's happen il' there were not 

 contained within these cells special formative forces which determine 

 them not only in their form and the rest "I* their constitution, l»ut 

 above all in their power of multiplication ' No special external 

 stimulus affects the still unfinished knob of 1 1 1 « - fly's leg unless it be 

 the removal of pressure; but this operates regularly, and cannot be 



