THE GERM-PLASM THEORY 365 



bristles, the odoriferous apparatus, and, in short, the whole complex 

 structure of the wing, with all its specific adaptations to the mode 

 of flight, to the manner of life, and to the colour of the environment. 

 How is it possible that all this can develop from a skin-cell? Is 

 it the influence of position that effects it, and could any other cell 

 of the caterpillar's skin do the same if it were placed in the same 

 position ? Could any neighbour-cell of the primitive wing-cell replace 

 it if it were destroyed? It is hardly probable, and I think I can 

 even prove that this is not so. The experiment of killing such 

 a cell in the living animal, has not yet been made; if it should 

 succeed, we may venture to say in advance that none of the neigh- 

 bouring skin-cells will be able to do its work and take its place 

 in developing a wing; the wing in question will simply remain 

 undeveloped. In the summer of 1897 I hatched a specimen of 

 Vanessa antiope from the pupa, which, though otherwise normal 

 and well-developed, lacked the left posterior wing altogether; no 

 trace of it could be recognized. In this case, from some cause 

 which could no longer be discovered, the first formative cell of 

 the wing in the hypodermis, or its descendants, must have been 

 destroyed, and no substitution of another took place, as the defect 

 showed. 



The young science of developmental mechanics attributes to 

 the position of a cell in the midst of a group of cells a determining 

 value as regards its further fate, and as far as the cells of the 

 segmenting ovum are concerned this seems to be true in certain 

 cases, but the assumption cannot be generally true except in a 

 very subordinate sense. The formative cell of the wing does not 

 become what it is because of its relative position in the organism. 

 If this were so it could not happen that a wing should develop 

 instead of a leg, as was observed in a Zygmna, nor could there be 

 any of those deformities already referred to, to which the name 

 'Heterotopia' is applied, and which consist in the development of 

 organs of definite normal structure, or at any rate of apparently 

 normal structure in quite unusual places, e. g. an antenna on the coxa 

 of a leg, or of a leg instead of an antenna (in Sirex), or instead of 

 a wing. It is therefore not some influence from without that makes 

 that particular skin-cell of the caterpillar the rudiment^ of the wing, 

 but the reasm lies within itself, in its own constitution. As the 

 whole mass of determinants for the whole body and for all the stages 

 of its development must be contained within the ovum and the 

 sperm-cell, so the primitive cell of the butterfly's wing must contain 

 all the determinants for the building up of this complicated part; 



