RELATION OF PHYSIOLOGY TO PHYSICS. 7 



follow the direction in which our assumption leads. In 

 assuming that the body is an enormously complex 

 physico-chemical structure, we have only begun to face 

 the difficulties of our hypothesis : for we have still to 

 consider how this structure can have originated in accord- 

 ance with the physico-chemical theory of life. The adult 

 organism develops from a single cell, the fertilised ovum. 

 It is certain that this cell does not contain in a preformed 

 condition the structure of an adult organism. The con- 

 ditions of environment in which any particular ovum 

 develops itself are doubtless indefinitely complex from 

 the physico-chemical standpoint, as indeed is the en- 

 vironment of any particular portion of matter existing 

 anywhere. But these conditions also vary almost in- 

 definitely in the case of different ova, whereas the adult 

 organism to which the ovum gives rise reproduces in 

 minute detail the enormously complex characters of the 

 parent organism. We are thus driven to the assumption 

 that the ovum contains .within itself a structure which, 

 given certain relatively simple conditions in the environ- 

 ment, reacts in such a way as to build up step by step, 

 from materials in the environment, the structure of the 

 adult organism. To effect this the germ-cell must have 

 a structure almost infinitely more definite and complex 

 than that of any cell in the adult organism. Difficult as 

 it may be to form any conception of the mechanism of a 

 secreting cell, it is infinitely more difficult to form the 

 remotest idea of that of a germ-cell. 



But we are still only at the beginning of the difficulty. 

 The assumed tremendous mechanism of the germ-cell 

 has been developed, together with the whole of the rest 

 of the parent organism and countless other germ-cells, 



