THE VITAL IMPETUS 133 



complex of determinants, and in the marshalling into 

 place of the isolated elements. The cell body — the 

 cytoplasm — carried out a very subordinate role, mainly 

 that of nourishing the essential chromatic substance. 

 Such was the Roux-Weismann Mosaic-theory of 

 development in its pristine form. 



It is clearly a preformation hypothesis. It is true 

 that the actual organism is not contained in the germ, 

 but all the parts of the latter, even the colours of the 

 eyes or hair, are present in it in the form of the de- 

 terminants. Obviously it involves a mechanism of 

 almost incredible complexity. But if we regard it as 

 a working hypothesis of development this complexity 

 of detail does not matter ; its truth would be indicated 

 by the fact that all analysis of the processes involved 

 would tend to simplify it and to smooth out the com- 

 plexity. But this is exactly what has not happened, 

 for all subsequent investigation has necessitated sub- 

 sidiary hypothesis after hypothesis. As a theory of 

 development it has failed entirely. 



If, after one of the blastomeres in the frog's egg at 

 the 2-cell stage be killed, the egg is then turned upside 

 down, the results of the experiment become totally 

 different ; the uninjured blast omere develops into a 

 whole embryo, differing from the normal one chiefly in 

 that it is smaller. If the uninjured egg in the 2-cell 

 stage be turned upside down two whole embryos, con- 

 nected together in various ways, develop. In the 

 frog's egg the two first blastomeres cannot be separated 

 from each other without rupturing them, but in the 

 egg of the salamander they can be separated. After 

 this separation two perfect, but small, embryos develop. 

 In the egg of the newt a fine thread can be tied round 

 the furrow formed by the first division. If this ligature 

 be tied loosely it does not affect development, and then 



