THE GERM-PLASM THEORY 



395 



pressure previously exerted ou the cell by its surroundings, the 

 material cause of a structure is to be sought for not in these condi. 

 tions of its appearance, but in the primary constituents which have 

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

 other words, through its determinants. How, for instance, could the 

 blunt rounded knob of the rough and clumsily jointed sac of cells 

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

 (Fig. 93, A) be incited to thicken, to constrict at the root (B), and 

 to form a joint-surface, to broaden out at the end, and produce 



Fig. 93. The development of a limb in the pupa of a Fly (Sarmphaga 

 carnaria). ' jl, apex of the limb from apupa four days old; thejointiiig is hinted 

 at ; hy, hypodermis ; ps, pupal sheath ; ph, phagocytes ; tr, tracheal branch. B, 

 the same on the fifth day ; the lumen of the limb is quite filled with phagocytes 

 (p/i) ; the last tarsal joint (t^) is beginning to show a bifid apex. C, the same on 

 the seventh, day ; the claws (Kr) and the adhesive lobes (Jil) are formed. 



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

 (Jcr), while beneath these a broad flat lobe {hi) grows forward, and 

 with its regularly disposed cells gradually forms the characteristic 

 adhesive organ of the fly — how could all this happen if there were not 

 contained within these cells special formative forces which determine 

 them not only in their form and the rest of their constitution, but 

 above all in their power of multiplication? No special external 

 stimulus aflFects the still unfinished knob of the fly's leg unless it be 

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



