186 LIFE: ITS NATURE AND ORIGIN 



egg and simply grew larger. Experimental confirmation of Roux' 

 view seemed evident when, in 1888, he reported that if one of the 

 first two blastomeres in the developing frog's egg was killed by a 

 hot needle, the remaining cell developed only a half-embryo. 



But in 1891 H. Driesch separated the first two blastomeres of a 

 sea-urchin's egg and was astonished to find that each cell formed 

 a normal though smaller gastrula and larva. The next year he 

 found that even in the four-cell stage, the single cells could pro- 

 duce complete individuals, as could also two eggs fused into each 

 other. It thus became evident that, initially, what was to happen 

 to any part of the egg was undetermined, even though if undis- 

 turbed it had a "prospective significance" (prospektive Bedeu- 

 tung). The fact that any part of the egg could be made to de- 

 velop quite differently from its normal expectation was due to 

 its multiple potency (pluripotency), and the term "prospective 

 potency" (prospektive Potenz) covered the total possibilities of 

 the part, irrespective of what might happen to it in any particular 

 experiment. 



Driesch was so impressed on finding in the egg this mysterious 

 ability to carry on, for which no material explanation seemed pos- 

 sible, that he invoked a metaphysical vital force or entelechy, to 

 account for it, and actually abandoned experimental science to 

 follow philosophical speculation. Notwithstanding this, we be- 

 lieve that the directing vital force is real enough, and that it is 

 explainable on the basis of comparatively simple physicoehemical 

 principles, chief of which is catalysis. 



Comparative Experimental Embryology 



Comparative experimental embryology, as contrasted with ob- 

 servations of naturally developing morphology, received a great 

 stimulus about 1897 when Gustav Born observed the remarkable 

 healing powers of amphibian embryos, and devised experimental 

 techniques which his early death prevented him from utilizing. 

 Fortunately, H. Spemann (Nobel Prize, 1935) made brilliant ex- 

 tensions of this work. With newts it was found that the develop- 

 mental fates of most embryonic regions are not irrevocably 

 determined until after a certain stage of gastrulation. Thus if a 

 bit of presumptive tube material from one embryo was trans- 

 planted to the presumptive gill region of another embryo, it 

 developed into gills, not neural tube. Presumptive skin, if trans- 

 planted into another embryo at a suitable region in what would 



