DEVELOPMENTAL MECHANICS. 167 



rately speaking, therefore, we should only have to determine 

 in respect of time, within what preceding developmental phases 

 structures which are not visible till sometime afterward can no 

 longer be varied by disturbing influences ; and in respect of 

 form, what preceding visible or invisible structures condition 

 every formation that is later observable, as, e.g., in the case 

 of the median sagittal plane of the embryo which is normally 

 conditioned by the first cleavage plane, and this in turn by the 

 axis of the copulation of the male and female pronuclei. 



Ultimately we shall attempt to get at the catisal modi op- 

 erandi, by attempting to ascertain their quality, and to trace out 

 the more general modi operandi, of the combination of which a 

 given effect is itself only a special case. 



For all this analytical experiment gives us ample opportunity. 

 By isolating, transposing, destroying, weakening, stimulating, 

 false union, passive deformation, changing the diet and the 

 functional size of the parts of eggs, embryos, or more devel- 

 oped organisms, by the application of unaccustomed agencies 

 like light, heat, electricity, and by the withdrawal of customary 

 influences, we may be able to ascertain a great many formative 

 operations in the parts of organisms. Thus we may, perhaps, 

 determine the possible influence of the muscles in the forma- 

 tion of the joints and -sockets by cutting the sinews of the 

 biceps and triceps brachii in very young animals and sewing 

 them on again with transposed insertions ; by cutting out 

 transverse wedge-shaped pieces from the longer bones and 

 feeding with madder, it may be possible to learn something of 

 the processes of functional adaptation in the structure of the 

 bones and hence of their immediate relations. 



By such artificial interference we shall in the first place be 

 able to establish the occurrence of dependent differentiation and 

 hence of differentiating reciprocal effects in such parts as are 

 far enough removed from one another to be isolated by the 

 crude means at our disposal, without their vitality being de- 

 stroyed by the harmful vicinity of the wounded region. 



Even now several results seem to show that during the 

 course of normal development, the "specific causes" of many 

 differentiations lie almost entirely within the altered parts, even 



