578 ON INCREASE IN COMPLEXITY [pt. iii 



at right angles to the host embryo if the organiser is trans- 

 planted into it at right angles. It must also have some sort of 

 "laterality", as has been proved by Goerttler *. Roux long ago con- 

 cluded from work with half-embryos that each lateral half could 

 develop more or less independently of the other, and Vogt, by 

 subjecting salamander eggs to a temperature gradient by holding 

 them in a silver plate on each side of which water at different 

 temperatures was circulating, found just the same thing. The left 

 half could be made to hypertrophy in relation to the right half or 

 vice versa. Huxley made very similar experiments with a less abrupt 

 temperature gradient. These experiments showed that the embryo 

 as a whole has a half-structure. But Goerttler went further in re- 

 moving the left side of a dorsal lip and grafting a right side, as it 

 were, from another embryo into it; whereupon two right halves of 

 a medullary plate and two right medullary folds were formed. Thus 

 the organiser itself must possess laterality. It has also what might 

 be called a regional structure, in that different parts of one organiser 

 tend to produce different embryonic structures. But the question is 

 an extremely complicated one, for a given part of the organising 

 region tends to induce more than it ought, and there are signs 

 that the term "harmonious equipotential system", which was first 

 applied by Driesch to totipotent blastomeres, may also be required 

 for certain parts of the organiser region at certain times. 



Of the physico-chemical nature of the organiser very little is 

 known. The small size of the amphibian material on which most 

 of these experiments have been made, and the body of experimental 

 difficulties as a whole, have made this side of the subject very back- 

 ward. Heteroplastic transplantations have shown, as we have seen, 

 that the organiser is not species-specific. It cannot act across a gap, 

 but requires continuity of the cell-mass for its effect. The inductive 

 power of the cells of the dorsal lip is not abolished by drying them, 

 according to Spemann, but freezing and thawing does lead to its loss. 

 Mangold has begun some interesting experiments on the quantitative 

 aspects of the working of the organiser. 



Whatever is the mechanism of the organiser, there is evidence that 

 it retains its activity unimpaired for a long time. Thus Mangold 



* Yet Spemann has found the inductive power still present in squashed pieces 

 from the organiser region. Does this mean that the laterality of the organiser is stereo- 

 chemical rather than cytological? 



